# Active Support Core Extensions¶ ↑

Active Support is the Ruby on Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff.

It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Ruby on Rails itself.

After reading this guide, you will know:

- What Core Extensions are.

- How to load all extensions.

- How to cherry-pick just the extensions you want.

- What extensions Active Support provides.

## How to Load Core Extensions¶ ↑

### Stand-Alone Active Support¶ ↑

In order to have a near-zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you can load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

    require 'active_support'

objects do not even respond to `blank?`. Let's see how to load its definition.

#### Cherry-picking a Definition¶ ↑

The most lightweight way to get `blank?` is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where such a method is defined. In the case of `blank?` the note reads:

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

That means that you can require it like this:

    require 'active_support'
    require 'active_support/core_ext/object/blank'

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

#### Loading Grouped Core Extensions¶ ↑

The next level is to simply load all extensions to `Object`. As a rule of thumb, extensions to `SomeClass` are available in one shot by loading `active_support/core_ext/some_class`.

Thus, to load all extensions to `Object` (including `blank?`):

    require 'active_support'
    require 'active_support/core_ext/object'

#### Loading All Core Extensions¶ ↑

You may prefer just to load all core extensions, there is a file for that:

    require 'active_support'
    require 'active_support/core_ext'

#### Loading All Active Support¶ ↑

And finally, if you want to have all Active Support available just issue:

    require 'active_support/all'

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via `autoload`, so it is only loaded if used.

### Active Support Within a Ruby on Rails Application¶ ↑

A Ruby on Rails application loads all Active Support unless `config.active_support.bare` is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

## Extensions to All Objects¶ ↑

### `blank?` and `present?`¶ ↑

The following values are considered to be blank in a Rails application:

- `nil` and `false`,

- strings composed only of whitespace (see note below),

- empty arrays and hashes, and

- any other object that responds to `empty?` and is empty.

INFO: The predicate for strings uses the Unicode-aware character class `[:space:]`, so for example U+2029 (paragraph separator) is considered to be whitespace.

WARNING: Note that numbers are not mentioned. In particular, 0 and 0.0 are not blank.

For example, this method from `ActionController::HttpAuthentication::Token::ControllerMethods` uses `blank?` for checking whether a token is present:

    def authenticate(controller, &login_procedure)
      token, options = token_and_options(controller.request)
      unless token.blank?
        login_procedure.call(token, options)
      end
    end

The method `present?` is equivalent to `!blank?`. This example is taken from `ActionDispatch::Http::Cache::Response`:

    def set_conditional_cache_control!
      return if self["Cache-Control"].present?
      ...
    end

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

### `presence`¶ ↑

The `presence` method returns its receiver if `present?`, and `nil` otherwise. It is useful for idioms like this:

    host = config[:host].presence || 'localhost'

NOTE: Defined in `active_support/core_ext/object/blank.rb`.

### `duplicable?`¶ ↑

A few fundamental objects in Ruby are singletons. For example, in the whole life of a program the integer 1 refers always to the same instance:

    1.object_id                 # => 3
    Math.cos(0).to_i.object_id  # => 3

Hence, there's no way these objects can be duplicated through `dup` or `clone`:

    true.dup  # => TypeError: can't dup TrueClass

Some numbers which are not singletons are not duplicable either:

    0.0.clone        # => allocator undefined for Float
    (2**1024).clone  # => allocator undefined for Bignum

Active Support provides `duplicable?` to programmatically query an object about this property:

    "foo".duplicable? # => true
    "".duplicable?    # => true
    0.0.duplicable?   # => false
    false.duplicable? # => false

By definition all objects are `duplicable?` except `nil`, `false`, `true`, symbols, numbers, class, module, and method objects.

WARNING: Any class can disallow duplication by removing `dup` and `clone` or raising exceptions from them. Thus only `rescue` can tell whether a given arbitrary object is duplicable. `duplicable?` depends on the hard-coded list above, but it is much faster than `rescue`. Use it only if you know the hard-coded list is enough in your use case.

NOTE: Defined in `active_support/core_ext/object/duplicable.rb`.

### `deep_dup`¶ ↑

The `deep_dup` method returns deep copy of a given object. Normally, when you `dup` an object that contains other objects, Ruby does not `dup` them, so it creates a shallow copy of the object. If you have an array with a string, for example, it will look like this:

    array     = ['string']
    duplicate = array.dup

    duplicate.push 'another-string'

    # the object was duplicated, so the element was added only to the duplicate
    array     # => ['string']
    duplicate # => ['string', 'another-string']

    duplicate.first.gsub!('string', 'foo')

    # first element was not duplicated, it will be changed in both arrays
    array     # => ['foo']
    duplicate # => ['foo', 'another-string']

As you can see, after duplicating the `Array` instance, we got another object, therefore we can modify it and the original object will stay unchanged. This is not true for array's elements, however. Since `dup` does not make deep copy, the string inside the array is still the same object.

If you need a deep copy of an object, you should use `deep_dup`. Here is an example:

    array     = ['string']
    duplicate = array.deep_dup

    duplicate.first.gsub!('string', 'foo')

    array     # => ['string']
    duplicate # => ['foo']

If the object is not duplicable, `deep_dup` will just return it:

    number = 1
    duplicate = number.deep_dup
    number.object_id == duplicate.object_id   # => true

NOTE: Defined in `active_support/core_ext/object/deep_dup.rb`.

### `try`¶ ↑

When you want to call a method on an object only if it is not `nil`, the simplest way to achieve it is with conditional statements, adding unnecessary clutter. The alternative is to use `try`. `try` is like `Object#send` except that it returns `nil` if sent to `nil`.

Here is an example:

    # without try
    unless @number.nil?
      @number.next
    end

    # with try
    @number.try(:next)

Another example is this code from `ActiveRecord::ConnectionAdapters::AbstractAdapter` where `@logger` could be `nil`. You can see that the code uses `try` and avoids an unnecessary check.

    def log_info(sql, name, ms)
      if @logger.try(:debug?)
        name = '%s (%.1fms)' % [name || 'SQL', ms]
        @logger.debug(format_log_entry(name, sql.squeeze(' ')))
      end
    end

`try` can also be called without arguments but a block, which will only be executed if the object is not nil:

    @person.try { |p| "#{p.first_name} #{p.last_name}" }

NOTE: Defined in `active_support/core_ext/object/try.rb`.

### `class_eval(*args, &block)`¶ ↑

You can evaluate code in the context of any object's singleton class using `class_eval`:

    class Proc
      def bind(object)
        block, time = self, Time.current
        object.class_eval do
          method_name = "__bind_#{time.to_i}_#{time.usec}"
          define_method(method_name, &block)
          method = instance_method(method_name)
          remove_method(method_name)
          method
        end.bind(object)
      end
    end

NOTE: Defined in `active_support/core_ext/kernel/singleton_class.rb`.

### `acts_like?(duck)`¶ ↑

The method `acts_like?` provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as `String` defines

    def acts_like_string?
    end

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:

    some_klass.acts_like?(:string)

Rails has classes that act like `Date` or `Time` and follow this contract.

NOTE: Defined in `active_support/core_ext/object/acts_like.rb`.

### `to_param`¶ ↑

All objects in Rails respond to the method `to_param`, which is meant to return something that represents them as values in a query string, or as URL fragments.

By default `to_param` just calls `to_s`:

    7.to_param # => "7"

The return value of `to_param` should not be escaped:

    "Tom & Jerry".to_param # => "Tom & Jerry"

Several classes in Rails overwrite this method.

For example `nil`, `true`, and `false` return themselves. `Array#to_param` calls `to_param` on the elements and joins the result with “/”:

    [0, true, String].to_param # => "0/true/String"

Notably, the Rails routing system calls `to_param` on models to get a value for the `:id` placeholder. `ActiveRecord::Base#to_param` returns the `id` of a model, but you can redefine that method in your models. For example, given

    class User
      def to_param
        "#{id}-#{name.parameterize}"
      end
    end

we get:

    user_path(@user) # => "/users/357-john-smith"

WARNING. Controllers need to be aware of any redefinition of `to_param` because when a request like that comes in “357-john-smith” is the value of `params[:id]`.

NOTE: Defined in `active_support/core_ext/object/to_param.rb`.

### `to_query`¶ ↑

Except for hashes, given an unescaped `key` this method constructs the part of a query string that would map such key to what `to_param` returns. For example, given

    class User
      def to_param
        "#{id}-#{name.parameterize}"
      end
    end

we get:

    current_user.to_query('user') # => user=357-john-smith

This method escapes whatever is needed, both for the key and the value:

    account.to_query('company[name]')
    # => "company%5Bname%5D=Johnson+%26+Johnson"

so its output is ready to be used in a query string.

Arrays return the result of applying `to_query` to each element with `key[]` as key, and join the result with “&”:

    [3.4, -45.6].to_query('sample')
    # => "sample%5B%5D=3.4&sample%5B%5D=-45.6"

Hashes also respond to `to_query` but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling `to_query(key)` on its values. Then it joins the result with “&”:

    {c: 3, b: 2, a: 1}.to_query # => "a=1&b=2&c=3"

The method `Hash#to_query` accepts an optional namespace for the keys:

    {id: 89, name: "John Smith"}.to_query('user')
    # => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"

NOTE: Defined in `active_support/core_ext/object/to_query.rb`.

### `with_options`¶ ↑

The method `with_options` provides a way to factor out common options in a series of method calls.

Given a default options hash, `with_options` yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

    class Account < ActiveRecord::Base
      has_many :customers, dependent: :destroy
      has_many :products,  dependent: :destroy
      has_many :invoices,  dependent: :destroy
      has_many :expenses,  dependent: :destroy
    end

this way:

    class Account < ActiveRecord::Base
      with_options dependent: :destroy do |assoc|
        assoc.has_many :customers
        assoc.has_many :products
        assoc.has_many :invoices
        assoc.has_many :expenses
      end
    end

That idiom may convey grouping to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

    I18n.with_options locale: user.locale, scope: "newsletter" do |i18n|
      subject i18n.t :subject
      body    i18n.t :body, user_name: user.name
    end

TIP: Since `with_options` forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

NOTE: Defined in `active_support/core_ext/object/with_options.rb`.

### JSON support¶ ↑

Active Support provides a better implementation of `to_json` than the `json` gem ordinarily provides for Ruby objects. This is because some classes, like `Hash`, `OrderedHash` and `Process::Status` need special handling in order to provide a proper JSON representation.

NOTE: Defined in `active_support/core_ext/object/json.rb`.

### Instance Variables¶ ↑

Active Support provides several methods to ease access to instance variables.

#### `instance_values`¶ ↑

The method `instance_values` returns a hash that maps instance variable names without “@” to their corresponding values. Keys are strings:

    class C
      def initialize(x, y)
        @x, @y = x, y
      end
    end

    C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}

NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.

#### `instance_variable_names`¶ ↑

The method `instance_variable_names` returns an array. Each name includes the “@” sign.

    class C
      def initialize(x, y)
        @x, @y = x, y
      end
    end

    C.new(0, 1).instance_variable_names # => ["@x", "@y"]

NOTE: Defined in `active_support/core_ext/object/instance_variables.rb`.

### Silencing Warnings, Streams, and Exceptions¶ ↑

The methods `silence_warnings` and `enable_warnings` change the value of `$VERBOSE` accordingly for the duration of their block, and reset it afterwards:

    silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }

You can silence any stream while a block runs with `silence_stream`:

    silence_stream(STDOUT) do
      # STDOUT is silent here
    end

The `quietly` method addresses the common use case where you want to silence STDOUT and STDERR, even in subprocesses:

    quietly { system 'bundle install' }

For example, the railties test suite uses that one in a few places to prevent command messages from being echoed intermixed with the progress status.

Silencing exceptions is also possible with `suppress`. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is `kind_of?` any of the arguments, `suppress` captures it and returns silently. Otherwise the exception is reraised:

    # If the user is locked the increment is lost, no big deal.
    suppress(ActiveRecord::StaleObjectError) do
      current_user.increment! :visits
    end

NOTE: Defined in `active_support/core_ext/kernel/reporting.rb`.

### `in?`¶ ↑

The predicate `in?` tests if an object is included in another object. An `ArgumentError` exception will be raised if the argument passed does not respond to `include?`.

Examples of `in?`:

    1.in?([1,2])        # => true
    "lo".in?("hello")   # => true
    25.in?(30..50)      # => false
    1.in?(1)            # => ArgumentError

NOTE: Defined in `active_support/core_ext/object/inclusion.rb`.

## Extensions to `Module`¶ ↑

### `alias_method_chain`¶ ↑

Using plain Ruby you can wrap methods with other methods, that's called alias chaining.

For example, let's say you'd like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap `ActionController::TestCase#process` this way in `test/test_helper.rb`:

    ActionController::TestCase.class_eval do
      # save a reference to the original process method
      alias_method :original_process, :process

      # now redefine process and delegate to original_process
      def process(action, params=nil, session=nil, flash=nil, http_method='GET')
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        original_process(action, params, session, flash, http_method)
      end
    end

That's the method `get`, `post`, etc., delegate the work to.

That technique has a risk, it could be the case that `:original_process` was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:

    ActionController::TestCase.class_eval do
      def process_with_stringified_params(...)
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        process_without_stringified_params(action, params, session, flash, http_method)
      end
      alias_method :process_without_stringified_params, :process
      alias_method :process, :process_with_stringified_params
    end

The method `alias_method_chain` provides a shortcut for that pattern:

    ActionController::TestCase.class_eval do
      def process_with_stringified_params(...)
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        process_without_stringified_params(action, params, session, flash, http_method)
      end
      alias_method_chain :process, :stringified_params
    end

Rails uses `alias_method_chain` all over the code base. For example validations are added to `ActiveRecord::Base#save` by wrapping the method that way in a separate module specialized in validations.

NOTE: Defined in `active_support/core_ext/module/aliasing.rb`.

### Attributes¶ ↑

#### `alias_attribute`¶ ↑

Model attributes have a reader, a writer, and a predicate. You can alias a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (one mnemonic is that they go in the same order as if you did an assignment):

    class User < ActiveRecord::Base
      # You can refer to the email column as "login".
      # This can be meaningful for authentication code.
      alias_attribute :login, :email
    end

NOTE: Defined in `active_support/core_ext/module/aliasing.rb`.

#### Internal Attributes¶ ↑

When you are defining an attribute in a class that is meant to be subclassed, name collisions are a risk. That's remarkably important for libraries.

Active Support defines the macros `attr_internal_reader`, `attr_internal_writer`, and `attr_internal_accessor`. They behave like their Ruby built-in `attr_*` counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro `attr_internal` is a synonym for `attr_internal_accessor`:

    # library
    class ThirdPartyLibrary::Crawler
      attr_internal :log_level
    end

    # client code
    class MyCrawler < ThirdPartyLibrary::Crawler
      attr_accessor :log_level
    end

In the previous example it could be the case that `:log_level` does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own `:log_level`. Thanks to `attr_internal` there's no collision.

By default the internal instance variable is named with a leading underscore, `@_log_level` in the example above. That's configurable via `Module.attr_internal_naming_format` though, you can pass any `sprintf`-like format string with a leading `@` and a `%s` somewhere, which is where the name will be placed. The default is `"@_%s"`.

Rails uses internal attributes in a few spots, for examples for views:

    module ActionView
      class Base
        attr_internal :captures
        attr_internal :request, :layout
        attr_internal :controller, :template
      end
    end

NOTE: Defined in `active_support/core_ext/module/attr_internal.rb`.

#### Module Attributes¶ ↑

The macros `mattr_reader`, `mattr_writer`, and `mattr_accessor` are the same as the `cattr_*` macros defined for class. In fact, the `cattr_*` macros are just aliases for the `mattr_*` macros. Check Class Attributes.

For example, the dependencies mechanism uses them:

    module ActiveSupport
      module Dependencies
        mattr_accessor :warnings_on_first_load
        mattr_accessor :history
        mattr_accessor :loaded
        mattr_accessor :mechanism
        mattr_accessor :load_paths
        mattr_accessor :load_once_paths
        mattr_accessor :autoloaded_constants
        mattr_accessor :explicitly_unloadable_constants
        mattr_accessor :logger
        mattr_accessor :log_activity
        mattr_accessor :constant_watch_stack
        mattr_accessor :constant_watch_stack_mutex
      end
    end

NOTE: Defined in `active_support/core_ext/module/attribute_accessors.rb`.

### Parents¶ ↑

#### `parent`¶ ↑

The `parent` method on a nested named module returns the module that contains its corresponding constant:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parent # => X::Y
    M.parent       # => X::Y

If the module is anonymous or belongs to the top-level, `parent` returns `Object`.

WARNING: Note that in that case `parent_name` returns `nil`.

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### `parent_name`¶ ↑

The `parent_name` method on a nested named module returns the fully-qualified name of the module that contains its corresponding constant:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parent_name # => "X::Y"
    M.parent_name       # => "X::Y"

For top-level or anonymous modules `parent_name` returns `nil`.

WARNING: Note that in that case `parent` returns `Object`.

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### `parents`¶ ↑

The method `parents` calls `parent` on the receiver and upwards until `Object` is reached. The chain is returned in an array, from bottom to top:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parents # => [X::Y, X, Object]
    M.parents       # => [X::Y, X, Object]

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

### Constants¶ ↑

The method `local_constants` returns the names of the constants that have been defined in the receiver module:

    module X
      X1 = 1
      X2 = 2
      module Y
        Y1 = :y1
        X1 = :overrides_X1_above
      end
    end

    X.local_constants    # => [:X1, :X2, :Y]
    X::Y.local_constants # => [:Y1, :X1]

The names are returned as symbols.

NOTE: Defined in `active_support/core_ext/module/introspection.rb`.

#### Qualified Constant Names¶ ↑

The standard methods `const_defined?`, `const_get` , and `const_set` accept bare constant names. Active Support extends this API to be able to pass relative qualified constant names.

The new methods are `qualified_const_defined?`, `qualified_const_get`, and `qualified_const_set`. Their arguments are assumed to be qualified constant names relative to their receiver:

    Object.qualified_const_defined?("Math::PI")       # => true
    Object.qualified_const_get("Math::PI")            # => 3.141592653589793
    Object.qualified_const_set("Math::Phi", 1.618034) # => 1.618034

Arguments may be bare constant names:

    Math.qualified_const_get("E") # => 2.718281828459045

These methods are analogous to their built-in counterparts. In particular, `qualified_constant_defined?` accepts an optional second argument to be able to say whether you want the predicate to look in the ancestors. This flag is taken into account for each constant in the expression while walking down the path.

For example, given

    module M
      X = 1
    end

    module N
      class C
        include M
      end
    end

`qualified_const_defined?` behaves this way:

    N.qualified_const_defined?("C::X", false) # => false
    N.qualified_const_defined?("C::X", true)  # => true
    N.qualified_const_defined?("C::X")        # => true

As the last example implies, the second argument defaults to true, as in `const_defined?`.

For coherence with the built-in methods only relative paths are accepted. Absolute qualified constant names like `::Math::PI` raise `NameError`.

NOTE: Defined in `active_support/core_ext/module/qualified_const.rb`.

### Reachable¶ ↑

A named module is reachable if it is stored in its corresponding constant. It means you can reach the module object via the constant.

That is what ordinarily happens, if a module is called “M”, the `M` constant exists and holds it:

    module M
    end

    M.reachable? # => true

But since constants and modules are indeed kind of decoupled, module objects can become unreachable:

    module M
    end

    orphan = Object.send(:remove_const, :M)

    # The module object is orphan now but it still has a name.
    orphan.name # => "M"

    # You cannot reach it via the constant M because it does not even exist.
    orphan.reachable? # => false

    # Let's define a module called "M" again.
    module M
    end

    # The constant M exists now again, and it stores a module
    # object called "M", but it is a new instance.
    orphan.reachable? # => false

NOTE: Defined in `active_support/core_ext/module/reachable.rb`.

### Anonymous¶ ↑

A module may or may not have a name:

    module M
    end
    M.name # => "M"

    N = Module.new
    N.name # => "N"

    Module.new.name # => nil

You can check whether a module has a name with the predicate `anonymous?`:

    module M
    end
    M.anonymous? # => false

    Module.new.anonymous? # => true

Note that being unreachable does not imply being anonymous:

    module M
    end

    m = Object.send(:remove_const, :M)

    m.reachable? # => false
    m.anonymous? # => false

though an anonymous module is unreachable by definition.

NOTE: Defined in `active_support/core_ext/module/anonymous.rb`.

### Method Delegation¶ ↑

The macro `delegate` offers an easy way to forward methods.

Let's imagine that users in some application have login information in the `User` model but name and other data in a separate `Profile` model:

    class User < ActiveRecord::Base
      has_one :profile
    end

With that configuration you get a user's name via their profile, `user.profile.name`, but it could be handy to still be able to access such attribute directly:

    class User < ActiveRecord::Base
      has_one :profile

      def name
        profile.name
      end
    end

That is what `delegate` does for you:

    class User < ActiveRecord::Base
      has_one :profile

      delegate :name, to: :profile
    end

It is shorter, and the intention more obvious.

The method must be public in the target.

The `delegate` macro accepts several methods:

    delegate :name, :age, :address, :twitter, to: :profile

When interpolated into a string, the `:to` option should become an expression that evaluates to the object the method is delegated to. Typically a string or symbol. Such an expression is evaluated in the context of the receiver:

    # delegates to the Rails constant
    delegate :logger, to: :Rails

    # delegates to the receiver's class
    delegate :table_name, to: :class

WARNING: If the `:prefix` option is `true` this is less generic, see below.

By default, if the delegation raises `NoMethodError` and the target is `nil` the exception is propagated. You can ask that `nil` is returned instead with the `:allow_nil` option:

    delegate :name, to: :profile, allow_nil: true

With `:allow_nil` the call `user.name` returns `nil` if the user has no profile.

The option `:prefix` adds a prefix to the name of the generated method. This may be handy for example to get a better name:

    delegate :street, to: :address, prefix: true

The previous example generates `address_street` rather than `street`.

WARNING: Since in this case the name of the generated method is composed of the target object and target method names, the `:to` option must be a method name.

A custom prefix may also be configured:

    delegate :size, to: :attachment, prefix: :avatar

In the previous example the macro generates `avatar_size` rather than `size`.

NOTE: Defined in `active_support/core_ext/module/delegation.rb`

### Redefining Methods¶ ↑

There are cases where you need to define a method with `define_method`, but don't know whether a method with that name already exists. If it does, a warning is issued if they are enabled. No big deal, but not clean either.

The method `redefine_method` prevents such a potential warning, removing the existing method before if needed.

NOTE: Defined in `active_support/core_ext/module/remove_method.rb`

## Extensions to `Class`¶ ↑

### Class Attributes¶ ↑

#### `class_attribute`¶ ↑

The method `class_attribute` declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.

    class A
      class_attribute :x
    end

    class B < A; end

    class C < B; end

    A.x = :a
    B.x # => :a
    C.x # => :a

    B.x = :b
    A.x # => :a
    C.x # => :b

    C.x = :c
    A.x # => :a
    B.x # => :b

For example `ActionMailer::Base` defines:

    class_attribute :default_params
    self.default_params = {
      mime_version: "1.0",
      charset: "UTF-8",
      content_type: "text/plain",
      parts_order: [ "text/plain", "text/enriched", "text/html" ]
    }.freeze

They can also be accessed and overridden at the instance level.

    A.x = 1

    a1 = A.new
    a2 = A.new
    a2.x = 2

    a1.x # => 1, comes from A
    a2.x # => 2, overridden in a2

The generation of the writer instance method can be prevented by setting the option `:instance_writer` to `false`.

    module ActiveRecord
      class Base
        class_attribute :table_name_prefix, instance_writer: false
        self.table_name_prefix = ""
      end
    end

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

The generation of the reader instance method can be prevented by setting the option `:instance_reader` to `false`.

    class A
      class_attribute :x, instance_reader: false
    end

    A.new.x = 1 # NoMethodError

For convenience `class_attribute` also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called `x?`.

When `:instance_reader` is `false`, the instance predicate returns a `NoMethodError` just like the reader method.

If you do not want the instance predicate, pass `instance_predicate: false` and it will not be defined.

NOTE: Defined in `active_support/core_ext/class/attribute.rb`

#### `cattr_reader`, `cattr_writer`, and `cattr_accessor`¶ ↑

The macros `cattr_reader`, `cattr_writer`, and `cattr_accessor` are analogous to their `attr_*` counterparts but for classes. They initialize a class variable to `nil` unless it already exists, and generate the corresponding class methods to access it:

    class MysqlAdapter < AbstractAdapter
      # Generates class methods to access @@emulate_booleans.
      cattr_accessor :emulate_booleans
      self.emulate_booleans = true
    end

Instance methods are created as well for convenience, they are just proxies to the class attribute. So, instances can change the class attribute, but cannot override it as it happens with `class_attribute` (see above). For example given

    module ActionView
      class Base
        cattr_accessor :field_error_proc
        @@field_error_proc = Proc.new{ ... }
      end
    end

we can access `field_error_proc` in views.

Also, you can pass a block to `cattr_*` to set up the attribute with a default value:

    class MysqlAdapter < AbstractAdapter
      # Generates class methods to access @@emulate_booleans with default value of true.
      cattr_accessor(:emulate_booleans) { true }
    end

The generation of the reader instance method can be prevented by setting `:instance_reader` to `false` and the generation of the writer instance method can be prevented by setting `:instance_writer` to `false`. Generation of both methods can be prevented by setting `:instance_accessor` to `false`. In all cases, the value must be exactly `false` and not any false value.

    module A
      class B
        # No first_name instance reader is generated.
        cattr_accessor :first_name, instance_reader: false
        # No last_name= instance writer is generated.
        cattr_accessor :last_name, instance_writer: false
        # No surname instance reader or surname= writer is generated.
        cattr_accessor :surname, instance_accessor: false
      end
    end

A model may find it useful to set `:instance_accessor` to `false` as a way to prevent mass-assignment from setting the attribute.

NOTE: Defined in `active_support/core_ext/module/attribute_accessors.rb`.

### Subclasses & Descendants¶ ↑

#### `subclasses`¶ ↑

The `subclasses` method returns the subclasses of the receiver:

    class C; end
    C.subclasses # => []

    class B < C; end
    C.subclasses # => [B]

    class A < B; end
    C.subclasses # => [B]

    class D < C; end
    C.subclasses # => [B, D]

The order in which these classes are returned is unspecified.

NOTE: Defined in `active_support/core_ext/class/subclasses.rb`.

#### `descendants`¶ ↑

The `descendants` method returns all classes that are `<` than its receiver:

    class C; end
    C.descendants # => []

    class B < C; end
    C.descendants # => [B]

    class A < B; end
    C.descendants # => [B, A]

    class D < C; end
    C.descendants # => [B, A, D]

The order in which these classes are returned is unspecified.

NOTE: Defined in `active_support/core_ext/class/subclasses.rb`.

## Extensions to `String`¶ ↑

### Output Safety¶ ↑

#### Motivation¶ ↑

Inserting data into HTML templates needs extra care. For example, you can't just interpolate `@review.title` verbatim into an HTML page. For one thing, if the review title is “Flanagan & Matz rules!” the output won't be well-formed because an ampersand has to be escaped as “&amp;”. What's more, depending on the application, that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the Security guide for further information about the risks.

#### Safe Strings¶ ↑

Active Support has the concept of (html) safe strings. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be unsafe by default:

    "".html_safe? # => false

You can obtain a safe string from a given one with the `html_safe` method:

    s = "".html_safe
    s.html_safe? # => true

It is important to understand that `html_safe` performs no escaping whatsoever, it is just an assertion:

    s = "<script>...</script>".html_safe
    s.html_safe? # => true
    s            # => "<script>...</script>"

It is your responsibility to ensure calling `html_safe` on a particular string is fine.

If you append onto a safe string, either in-place with `concat`/`<<`, or with `+`, the result is a safe string. Unsafe arguments are escaped:

    "".html_safe + "<" # => "&lt;"

Safe arguments are directly appended:

    "".html_safe + "<".html_safe # => "<"

These methods should not be used in ordinary views. Unsafe values are automatically escaped:

    <%= @review.title %> <%# fine, escaped if needed %>

To insert something verbatim use the `raw` helper rather than calling `html_safe`:

    <%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>

or, equivalently, use `<%==`:

    <%== @cms.current_template %> <%# inserts @cms.current_template as is %>

The `raw` helper calls `html_safe` for you:

    def raw(stringish)
      stringish.to_s.html_safe
    end

NOTE: Defined in `active_support/core_ext/string/output_safety.rb`.

#### Transformation¶ ↑

As a rule of thumb, except perhaps for concatenation as explained above, any method that may change a string gives you an unsafe string. These are `downcase`, `gsub`, `strip`, `chomp`, `underscore`, etc.

In the case of in-place transformations like `gsub!` the receiver itself becomes unsafe.

INFO: The safety bit is lost always, no matter whether the transformation actually changed something.

#### Conversion and Coercion¶ ↑

Calling `to_s` on a safe string returns a safe string, but coercion with `to_str` returns an unsafe string.

#### Copying¶ ↑

Calling `dup` or `clone` on safe strings yields safe strings.

### `remove`¶ ↑

The method `remove` will remove all occurrences of the pattern:

    "Hello World".remove(/Hello /) => "World"

There's also the destructive version `String#remove!`.

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `squish`¶ ↑

The method `squish` strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

    " \n  foo\n\r \t bar \n".squish # => "foo bar"

There's also the destructive version `String#squish!`.

Note that it handles both ASCII and Unicode whitespace.

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `truncate`¶ ↑

The method `truncate` returns a copy of its receiver truncated after a given `length`:

    "Oh dear! Oh dear! I shall be late!".truncate(20)
    # => "Oh dear! Oh dear!..."

Ellipsis can be customized with the `:omission` option:

    "Oh dear! Oh dear! I shall be late!".truncate(20, omission: '&hellip;')
    # => "Oh dear! Oh &hellip;"

Note in particular that truncation takes into account the length of the omission string.

Pass a `:separator` to truncate the string at a natural break:

    "Oh dear! Oh dear! I shall be late!".truncate(18)
    # => "Oh dear! Oh dea..."
    "Oh dear! Oh dear! I shall be late!".truncate(18, separator: ' ')
    # => "Oh dear! Oh..."

The option `:separator` can be a regexp:

    "Oh dear! Oh dear! I shall be late!".truncate(18, separator: /\s/)
    # => "Oh dear! Oh..."

In above examples “dear” gets cut first, but then `:separator` prevents it.

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `truncate_words`¶ ↑

The method `truncate_words` returns a copy of its receiver truncated after a given number of words:

    "Oh dear! Oh dear! I shall be late!".truncate_words(4)
    # => "Oh dear! Oh dear!..."

Ellipsis can be customized with the `:omission` option:

    "Oh dear! Oh dear! I shall be late!".truncate_words(4, omission: '&hellip;')
    # => "Oh dear! Oh dear!&hellip;"

Pass a `:separator` to truncate the string at a natural break:

    "Oh dear! Oh dear! I shall be late!".truncate_words(3, separator: '!')
    # => "Oh dear! Oh dear! I shall be late..."

The option `:separator` can be a regexp:

    "Oh dear! Oh dear! I shall be late!".truncate_words(4, separator: /\s/)
    # => "Oh dear! Oh dear!..."

NOTE: Defined in `active_support/core_ext/string/filters.rb`.

### `inquiry`¶ ↑

The `inquiry` method converts a string into a `StringInquirer` object making equality checks prettier.

    "production".inquiry.production? # => true
    "active".inquiry.inactive?       # => false

### `starts_with?` and `ends_with?`¶ ↑

Active Support defines 3rd person aliases of `String#start_with?` and `String#end_with?`:

    "foo".starts_with?("f") # => true
    "foo".ends_with?("o")   # => true

NOTE: Defined in `active_support/core_ext/string/starts_ends_with.rb`.

### `strip_heredoc`¶ ↑

The method `strip_heredoc` strips indentation in heredocs.

For example in

    if options[:usage]
      puts "    This command does such and such.

        Supported options are:
          -h         This message
          ...
    ".strip_heredoc
    end

the user would see the usage message aligned against the left margin.

Technically, it looks for the least indented line in the whole string, and removes that amount of leading whitespace.

NOTE: Defined in `active_support/core_ext/string/strip.rb`.

### `indent`¶ ↑

Indents the lines in the receiver:

    "def some_method
      some_code
    end
    ".indent(2)
    # =>
      def some_method
        some_code
      end

The second argument, `indent_string`, specifies which indent string to use. The default is `nil`, which tells the method to make an educated guess peeking at the first indented line, and fallback to a space if there is none.

    "  foo".indent(2)        # => "    foo"
    "foo\n\t\tbar".indent(2) # => "\t\tfoo\n\t\t\t\tbar"
    "foo".indent(2, "\t")    # => "\t\tfoo"

While `indent_string` is typically one space or tab, it may be any string.

The third argument, `indent_empty_lines`, is a flag that says whether empty lines should be indented. Default is false.

    "foo\n\nbar".indent(2)            # => "  foo\n\n  bar"
    "foo\n\nbar".indent(2, nil, true) # => "  foo\n  \n  bar"

The `indent!` method performs indentation in-place.

NOTE: Defined in `active_support/core_ext/string/indent.rb`.

### Access¶ ↑

#### `at(position)`¶ ↑

Returns the character of the string at position `position`:

    "hello".at(0)  # => "h"
    "hello".at(4)  # => "o"
    "hello".at(-1) # => "o"
    "hello".at(10) # => nil

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `from(position)`¶ ↑

Returns the substring of the string starting at position `position`:

    "hello".from(0)  # => "hello"
    "hello".from(2)  # => "llo"
    "hello".from(-2) # => "lo"
    "hello".from(10) # => "" if < 1.9, nil in 1.9

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `to(position)`¶ ↑

Returns the substring of the string up to position `position`:

    "hello".to(0)  # => "h"
    "hello".to(2)  # => "hel"
    "hello".to(-2) # => "hell"
    "hello".to(10) # => "hello"

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `first(limit = 1)`¶ ↑

The call `str.first(n)` is equivalent to `str.to(n-1)` if `n` > 0, and returns an empty string for `n` == 0.

NOTE: Defined in `active_support/core_ext/string/access.rb`.

#### `last(limit = 1)`¶ ↑

The call `str.last(n)` is equivalent to `str.from(-n)` if `n` > 0, and returns an empty string for `n` == 0.

NOTE: Defined in `active_support/core_ext/string/access.rb`.

### Inflections¶ ↑

#### `pluralize`¶ ↑

The method `pluralize` returns the plural of its receiver:

    "table".pluralize     # => "tables"
    "ruby".pluralize      # => "rubies"
    "equipment".pluralize # => "equipment"

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in `config/initializers/inflections.rb`. That file is generated by the `rails` command and has instructions in comments.

`pluralize` can also take an optional `count` parameter. If `count == 1` the singular form will be returned. For any other value of `count` the plural form will be returned:

    "dude".pluralize(0) # => "dudes"
    "dude".pluralize(1) # => "dude"
    "dude".pluralize(2) # => "dudes"

Active Record uses this method to compute the default table name that corresponds to a model:

    # active_record/model_schema.rb
    def undecorated_table_name(class_name = base_class.name)
      table_name = class_name.to_s.demodulize.underscore
      pluralize_table_names ? table_name.pluralize : table_name
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `singularize`¶ ↑

The inverse of `pluralize`:

    "tables".singularize    # => "table"
    "rubies".singularize    # => "ruby"
    "equipment".singularize # => "equipment"

Associations compute the name of the corresponding default associated class using this method:

    # active_record/reflection.rb
    def derive_class_name
      class_name = name.to_s.camelize
      class_name = class_name.singularize if collection?
      class_name
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `camelize`¶ ↑

The method `camelize` returns its receiver in camel case:

    "product".camelize    # => "Product"
    "admin_user".camelize # => "AdminUser"

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

    "backoffice/session".camelize # => "Backoffice::Session"

For example, Action Pack uses this method to load the class that provides a certain session store:

    # action_controller/metal/session_management.rb
    def session_store=(store)
      @@session_store = store.is_a?(Symbol) ?
        ActionDispatch::Session.const_get(store.to_s.camelize) :
        store
    end

`camelize` accepts an optional argument, it can be `:upper` (default), or `:lower`. With the latter the first letter becomes lowercase:

    "visual_effect".camelize(:lower) # => "visualEffect"

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

INFO: As a rule of thumb you can think of `camelize` as the inverse of `underscore`, though there are cases where that does not hold: `"SSLError".underscore.camelize` gives back `"SslError"`. To support cases such as this, Active Support allows you to specify acronyms in `config/initializers/inflections.rb`:

    ActiveSupport::Inflector.inflections do |inflect|
      inflect.acronym 'SSL'
    end

    "SSLError".underscore.camelize # => "SSLError"

`camelize` is aliased to `camelcase`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `underscore`¶ ↑

The method `underscore` goes the other way around, from camel case to paths:

    "Product".underscore   # => "product"
    "AdminUser".underscore # => "admin_user"

Also converts “::” back to “/”:

    "Backoffice::Session".underscore # => "backoffice/session"

and understands strings that start with lowercase:

    "visualEffect".underscore # => "visual_effect"

`underscore` accepts no argument though.

Rails class and module autoloading uses `underscore` to infer the relative path without extension of a file that would define a given missing constant:

    # active_support/dependencies.rb
    def load_missing_constant(from_mod, const_name)
      ...
      qualified_name = qualified_name_for from_mod, const_name
      path_suffix = qualified_name.underscore
      ...
    end

INFO: As a rule of thumb you can think of `underscore` as the inverse of `camelize`, though there are cases where that does not hold. For example, `"SSLError".underscore.camelize` gives back `"SslError"`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `titleize`¶ ↑

The method `titleize` capitalizes the words in the receiver:

    "alice in wonderland".titleize # => "Alice In Wonderland"
    "fermat's enigma".titleize     # => "Fermat's Enigma"

`titleize` is aliased to `titlecase`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `dasherize`¶ ↑

The method `dasherize` replaces the underscores in the receiver with dashes:

    "name".dasherize         # => "name"
    "contact_data".dasherize # => "contact-data"

The XML serializer of models uses this method to dasherize node names:

    # active_model/serializers/xml.rb
    def reformat_name(name)
      name = name.camelize if camelize?
      dasherize? ? name.dasherize : name
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `demodulize`¶ ↑

Given a string with a qualified constant name, `demodulize` returns the very constant name, that is, the rightmost part of it:

    "Product".demodulize                        # => "Product"
    "Backoffice::UsersController".demodulize    # => "UsersController"
    "Admin::Hotel::ReservationUtils".demodulize # => "ReservationUtils"
    "::Inflections".demodulize                  # => "Inflections"
    "".demodulize                               # => ""

Active Record for example uses this method to compute the name of a counter cache column:

    # active_record/reflection.rb
    def counter_cache_column
      if options[:counter_cache] == true
        "#{active_record.name.demodulize.underscore.pluralize}_count"
      elsif options[:counter_cache]
        options[:counter_cache]
      end
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `deconstantize`¶ ↑

Given a string with a qualified constant reference expression, `deconstantize` removes the rightmost segment, generally leaving the name of the constant's container:

    "Product".deconstantize                        # => ""
    "Backoffice::UsersController".deconstantize    # => "Backoffice"
    "Admin::Hotel::ReservationUtils".deconstantize # => "Admin::Hotel"

Active Support for example uses this method in `Module#qualified_const_set`:

    def qualified_const_set(path, value)
      QualifiedConstUtils.raise_if_absolute(path)

      const_name = path.demodulize
      mod_name = path.deconstantize
      mod = mod_name.empty? ? self : qualified_const_get(mod_name)
      mod.const_set(const_name, value)
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `parameterize`¶ ↑

The method `parameterize` normalizes its receiver in a way that can be used in pretty URLs.

    "John Smith".parameterize # => "john-smith"
    "Kurt Gödel".parameterize # => "kurt-godel"

In fact, the result string is wrapped in an instance of `ActiveSupport::Multibyte::Chars`.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `tableize`¶ ↑

The method `tableize` is `underscore` followed by `pluralize`.

    "Person".tableize      # => "people"
    "Invoice".tableize     # => "invoices"
    "InvoiceLine".tableize # => "invoice_lines"

As a rule of thumb, `tableize` returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight `tableize` indeed, because it also demodulizes the class name and checks a few options that may affect the returned string.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `classify`¶ ↑

The method `classify` is the inverse of `tableize`. It gives you the class name corresponding to a table name:

    "people".classify        # => "Person"
    "invoices".classify      # => "Invoice"
    "invoice_lines".classify # => "InvoiceLine"

The method understands qualified table names:

    "highrise_production.companies".classify # => "Company"

Note that `classify` returns a class name as a string. You can get the actual class object invoking `constantize` on it, explained next.

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `constantize`¶ ↑

The method `constantize` resolves the constant reference expression in its receiver:

    "Fixnum".constantize # => Fixnum

    module M
      X = 1
    end
    "M::X".constantize # => 1

If the string evaluates to no known constant, or its content is not even a valid constant name, `constantize` raises `NameError`.

Constant name resolution by `constantize` starts always at the top-level `Object` even if there is no leading “::”.

    X = :in_Object
    module M
      X = :in_M

      X                 # => :in_M
      "::X".constantize # => :in_Object
      "X".constantize   # => :in_Object (!)
    end

So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using `constantize`:

    # action_mailer/test_case.rb
    def determine_default_mailer(name)
      name.sub(/Test$/, '').constantize
    rescue NameError => e
      raise NonInferrableMailerError.new(name)
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `humanize`¶ ↑

The method `humanize` tweaks an attribute name for display to end users.

Specifically performs these transformations:

- Applies human inflection rules to the argument.

- Deletes leading underscores, if any.

- Removes a “_id” suffix if present.

- Replaces underscores with spaces, if any.

- Downcases all words except acronyms.

- Capitalizes the first word.

The capitalization of the first word can be turned off by setting the `:capitalize` option to false (default is true).

    "name".humanize                         # => "Name"
    "author_id".humanize                    # => "Author"
    "author_id".humanize(capitalize: false) # => "author"
    "comments_count".humanize               # => "Comments count"
    "_id".humanize                          # => "Id"

If “SSL” was defined to be an acronym:

    'ssl_error'.humanize # => "SSL error"

The helper method `full_messages` uses `humanize` as a fallback to include attribute names:

    def full_messages
      map { |attribute, message| full_message(attribute, message) }
    end

    def full_message
      ...
      attr_name = attribute.to_s.tr('.', '_').humanize
      attr_name = @base.class.human_attribute_name(attribute, default: attr_name)
      ...
    end

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

#### `foreign_key`¶ ↑

The method `foreign_key` gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds “_id”:

    "User".foreign_key           # => "user_id"
    "InvoiceLine".foreign_key    # => "invoice_line_id"
    "Admin::Session".foreign_key # => "session_id"

Pass a false argument if you do not want the underscore in “_id”:

    "User".foreign_key(false) # => "userid"

Associations use this method to infer foreign keys, for example `has_one` and `has_many` do this:

    # active_record/associations.rb
    foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key

NOTE: Defined in `active_support/core_ext/string/inflections.rb`.

### Conversions¶ ↑

#### `to_date`, `to_time`, `to_datetime`¶ ↑

The methods `to_date`, `to_time`, and `to_datetime` are basically convenience wrappers around `Date._parse`:

    "2010-07-27".to_date              # => Tue, 27 Jul 2010
    "2010-07-27 23:37:00".to_time     # => Tue Jul 27 23:37:00 UTC 2010
    "2010-07-27 23:37:00".to_datetime # => Tue, 27 Jul 2010 23:37:00 +0000

`to_time` receives an optional argument `:utc` or `:local`, to indicate which time zone you want the time in:

    "2010-07-27 23:42:00".to_time(:utc)   # => Tue Jul 27 23:42:00 UTC 2010
    "2010-07-27 23:42:00".to_time(:local) # => Tue Jul 27 23:42:00 +0200 2010

Default is `:utc`.

Please refer to the documentation of `Date._parse` for further details.

INFO: The three of them return `nil` for blank receivers.

NOTE: Defined in `active_support/core_ext/string/conversions.rb`.

## Extensions to `Numeric`¶ ↑

### Bytes¶ ↑

All numbers respond to these methods:

    bytes
    kilobytes
    megabytes
    gigabytes
    terabytes
    petabytes
    exabytes

They return the corresponding amount of bytes, using a conversion factor of 1024:

    2.kilobytes   # => 2048
    3.megabytes   # => 3145728
    3.5.gigabytes # => 3758096384
    -4.exabytes   # => -4611686018427387904

Singular forms are aliased so you are able to say:

    1.megabyte # => 1048576

NOTE: Defined in `active_support/core_ext/numeric/bytes.rb`.

### Time¶ ↑

Enables the use of time calculations and declarations, like `45.minutes + 2.hours + 4.years`.

These methods use Time#advance for precise date calculations when using from_now, ago, etc. as well as adding or subtracting their results from a Time object. For example:

    # equivalent to Time.current.advance(months: 1)
    1.month.from_now

    # equivalent to Time.current.advance(years: 2)
    2.years.from_now

    # equivalent to Time.current.advance(months: 4, years: 5)
    (4.months + 5.years).from_now

### Formatting¶ ↑

Enables the formatting of numbers in a variety of ways.

Produce a string representation of a number as a telephone number:

    5551234.to_s(:phone)
    # => 555-1234
    1235551234.to_s(:phone)
    # => 123-555-1234
    1235551234.to_s(:phone, area_code: true)
    # => (123) 555-1234
    1235551234.to_s(:phone, delimiter: " ")
    # => 123 555 1234
    1235551234.to_s(:phone, area_code: true, extension: 555)
    # => (123) 555-1234 x 555
    1235551234.to_s(:phone, country_code: 1)
    # => +1-123-555-1234

Produce a string representation of a number as currency:

    1234567890.50.to_s(:currency)                 # => $1,234,567,890.50
    1234567890.506.to_s(:currency)                # => $1,234,567,890.51
    1234567890.506.to_s(:currency, precision: 3)  # => $1,234,567,890.506

Produce a string representation of a number as a percentage:

    100.to_s(:percentage)
    # => 100.000%
    100.to_s(:percentage, precision: 0)
    # => 100%
    1000.to_s(:percentage, delimiter: '.', separator: ',')
    # => 1.000,000%
    302.24398923423.to_s(:percentage, precision: 5)
    # => 302.24399%

Produce a string representation of a number in delimited form:

    12345678.to_s(:delimited)                     # => 12,345,678
    12345678.05.to_s(:delimited)                  # => 12,345,678.05
    12345678.to_s(:delimited, delimiter: ".")     # => 12.345.678
    12345678.to_s(:delimited, delimiter: ",")     # => 12,345,678
    12345678.05.to_s(:delimited, separator: " ")  # => 12,345,678 05

Produce a string representation of a number rounded to a precision:

    111.2345.to_s(:rounded)                     # => 111.235
    111.2345.to_s(:rounded, precision: 2)       # => 111.23
    13.to_s(:rounded, precision: 5)             # => 13.00000
    389.32314.to_s(:rounded, precision: 0)      # => 389
    111.2345.to_s(:rounded, significant: true)  # => 111

Produce a string representation of a number as a human-readable number of bytes:

    123.to_s(:human_size)            # => 123 Bytes
    1234.to_s(:human_size)           # => 1.21 KB
    12345.to_s(:human_size)          # => 12.1 KB
    1234567.to_s(:human_size)        # => 1.18 MB
    1234567890.to_s(:human_size)     # => 1.15 GB
    1234567890123.to_s(:human_size)  # => 1.12 TB

Produce a string representation of a number in human-readable words:

    123.to_s(:human)               # => "123"
    1234.to_s(:human)              # => "1.23 Thousand"
    12345.to_s(:human)             # => "12.3 Thousand"
    1234567.to_s(:human)           # => "1.23 Million"
    1234567890.to_s(:human)        # => "1.23 Billion"
    1234567890123.to_s(:human)     # => "1.23 Trillion"
    1234567890123456.to_s(:human)  # => "1.23 Quadrillion"

NOTE: Defined in `active_support/core_ext/numeric/conversions.rb`.

## Extensions to `Integer`¶ ↑

### `multiple_of?`¶ ↑

The method `multiple_of?` tests whether an integer is multiple of the argument:

    2.multiple_of?(1) # => true
    1.multiple_of?(2) # => false

NOTE: Defined in `active_support/core_ext/integer/multiple.rb`.

### `ordinal`¶ ↑

The method `ordinal` returns the ordinal suffix string corresponding to the receiver integer:

    1.ordinal    # => "st"
    2.ordinal    # => "nd"
    53.ordinal   # => "rd"
    2009.ordinal # => "th"
    -21.ordinal  # => "st"
    -134.ordinal # => "th"

NOTE: Defined in `active_support/core_ext/integer/inflections.rb`.

### `ordinalize`¶ ↑

The method `ordinalize` returns the ordinal string corresponding to the receiver integer. In comparison, note that the `ordinal` method returns only the suffix string.

    1.ordinalize    # => "1st"
    2.ordinalize    # => "2nd"
    53.ordinalize   # => "53rd"
    2009.ordinalize # => "2009th"
    -21.ordinalize  # => "-21st"
    -134.ordinalize # => "-134th"

NOTE: Defined in `active_support/core_ext/integer/inflections.rb`.

## Extensions to `BigDecimal`¶ ↑

### `to_s`¶ ↑

The method `to_s` is aliased to `to_formatted_s`. This provides a convenient way to display a BigDecimal value in floating-point notation:

    BigDecimal.new(5.00, 6).to_s  # => "5.0"

### `to_formatted_s`¶ ↑

Te method `to_formatted_s` provides a default specifier of “F”. This means that a simple call to `to_formatted_s` or `to_s` will result in floating point representation instead of engineering notation:

    BigDecimal.new(5.00, 6).to_formatted_s  # => "5.0"

and that symbol specifiers are also supported:

    BigDecimal.new(5.00, 6).to_formatted_s(:db)  # => "5.0"

Engineering notation is still supported:

    BigDecimal.new(5.00, 6).to_formatted_s("e")  # => "0.5E1"

## Extensions to `Enumerable`¶ ↑

### `sum`¶ ↑

The method `sum` adds the elements of an enumerable:

    [1, 2, 3].sum # => 6
    (1..100).sum  # => 5050

Addition only assumes the elements respond to `+`:

    [[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
    %w(foo bar baz).sum             # => "foobarbaz"
    {a: 1, b: 2, c: 3}.sum # => [:b, 2, :c, 3, :a, 1]

The sum of an empty collection is zero by default, but this is customizable:

    [].sum    # => 0
    [].sum(1) # => 1

If a block is given, `sum` becomes an iterator that yields the elements of the collection and sums the returned values:

    (1..5).sum {|n| n * 2 } # => 30
    [2, 4, 6, 8, 10].sum    # => 30

The sum of an empty receiver can be customized in this form as well:

    [].sum(1) {|n| n**3} # => 1

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `index_by`¶ ↑

The method `index_by` generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:

    invoices.index_by(&:number)
    # => {'2009-032' => <Invoice ...>, '2009-008' => <Invoice ...>, ...}

WARNING. Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `many?`¶ ↑

The method `many?` is shorthand for `collection.size > 1`:

    <% if pages.many? %>
      <%= pagination_links %>
    <% end %>

If an optional block is given, `many?` only takes into account those elements that return true:

    @see_more = videos.many? {|video| video.category == params[:category]}

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

### `exclude?`¶ ↑

The predicate `exclude?` tests whether a given object does not belong to the collection. It is the negation of the built-in `include?`:

    to_visit << node if visited.exclude?(node)

NOTE: Defined in `active_support/core_ext/enumerable.rb`.

## Extensions to `Array`¶ ↑

### Accessing¶ ↑

Active Support augments the API of arrays to ease certain ways of accessing them. For example, `to` returns the subarray of elements up to the one at the passed index:

    %w(a b c d).to(2) # => %w(a b c)
    [].to(7)          # => []

Similarly, `from` returns the tail from the element at the passed index to the end. If the index is greater than the length of the array, it returns an empty array.

    %w(a b c d).from(2)  # => %w(c d)
    %w(a b c d).from(10) # => []
    [].from(0)           # => []

The methods `second`, `third`, `fourth`, and `fifth` return the corresponding element (`first` is built-in). Thanks to social wisdom and positive constructiveness all around, `forty_two` is also available.

    %w(a b c d).third # => c
    %w(a b c d).fifth # => nil

NOTE: Defined in `active_support/core_ext/array/access.rb`.

### Adding Elements¶ ↑

#### `prepend`¶ ↑

This method is an alias of `Array#unshift`.

    %w(a b c d).prepend('e')  # => %w(e a b c d)
    [].prepend(10)            # => [10]

NOTE: Defined in `active_support/core_ext/array/prepend_and_append.rb`.

#### `append`¶ ↑

This method is an alias of `Array#<<`.

    %w(a b c d).append('e')  # => %w(a b c d e)
    [].append([1,2])         # => [[1,2]]

NOTE: Defined in `active_support/core_ext/array/prepend_and_append.rb`.

### Options Extraction¶ ↑

When the last argument in a method call is a hash, except perhaps for a `&block` argument, Ruby allows you to omit the brackets:

    User.exists?(email: params[:email])

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses `*` in its declaration, however, such an options hash ends up being an item of the array of arguments, where it loses its role.

In those cases, you may give an options hash a distinguished treatment with `extract_options!`. This method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise it returns an empty hash.

Let's see for example the definition of the `caches_action` controller macro:

    def caches_action(*actions)
      return unless cache_configured?
      options = actions.extract_options!
      ...
    end

This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to `extract_options!` you obtain the options hash and remove it from `actions` in a simple and explicit way.

NOTE: Defined in `active_support/core_ext/array/extract_options.rb`.

### Conversions¶ ↑

#### `to_sentence`¶ ↑

The method `to_sentence` turns an array into a string containing a sentence that enumerates its items:

    %w().to_sentence                # => ""
    %w(Earth).to_sentence           # => "Earth"
    %w(Earth Wind).to_sentence      # => "Earth and Wind"
    %w(Earth Wind Fire).to_sentence # => "Earth, Wind, and Fire"

This method accepts three options:

- `:two_words_connector`: What is used for arrays of length 2. Default is “ and ”.

- `:words_connector`: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is “, ”.

- `:last_word_connector`: What is used to join the last items of an array with 3 or more elements. Default is “, and ”.

The defaults for these options can be localized, their keys are:

| Option | I18n key | | ———————- | ———————————– | | `:two_words_connector` | `support.array.two_words_connector` | | `:words_connector` | `support.array.words_connector` | | `:last_word_connector` | `support.array.last_word_connector` |

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

#### `to_formatted_s`¶ ↑

The method `to_formatted_s` acts like `to_s` by default.

If the array contains items that respond to `id`, however, the symbol `:db` may be passed as argument. That's typically used with collections of Active Record objects. Returned strings are:

    [].to_formatted_s(:db)            # => "null"
    [user].to_formatted_s(:db)        # => "8456"
    invoice.lines.to_formatted_s(:db) # => "23,567,556,12"

Integers in the example above are supposed to come from the respective calls to `id`.

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

#### `to_xml`¶ ↑

The method `to_xml` returns a string containing an XML representation of its receiver:

    Contributor.limit(2).order(:rank).to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <contributors type="array">
    #   <contributor>
    #     <id type="integer">4356</id>
    #     <name>Jeremy Kemper</name>
    #     <rank type="integer">1</rank>
    #     <url-id>jeremy-kemper</url-id>
    #   </contributor>
    #   <contributor>
    #     <id type="integer">4404</id>
    #     <name>David Heinemeier Hansson</name>
    #     <rank type="integer">2</rank>
    #     <url-id>david-heinemeier-hansson</url-id>
    #   </contributor>
    # </contributors>

To do so it sends `to_xml` to every item in turn, and collects the results under a root node. All items must respond to `to_xml`, an exception is raised otherwise.

By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with `is_a?`) and they are not hashes. In the example above that's “contributors”.

If there's any element that does not belong to the type of the first one the root node becomes “objects”:

    [Contributor.first, Commit.first].to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <objects type="array">
    #   <object>
    #     <id type="integer">4583</id>
    #     <name>Aaron Batalion</name>
    #     <rank type="integer">53</rank>
    #     <url-id>aaron-batalion</url-id>
    #   </object>
    #   <object>
    #     <author>Joshua Peek</author>
    #     <authored-timestamp type="datetime">2009-09-02T16:44:36Z</authored-timestamp>
    #     <branch>origin/master</branch>
    #     <committed-timestamp type="datetime">2009-09-02T16:44:36Z</committed-timestamp>
    #     <committer>Joshua Peek</committer>
    #     <git-show nil="true"></git-show>
    #     <id type="integer">190316</id>
    #     <imported-from-svn type="boolean">false</imported-from-svn>
    #     <message>Kill AMo observing wrap_with_notifications since ARes was only using it</message>
    #     <sha1>723a47bfb3708f968821bc969a9a3fc873a3ed58</sha1>
    #   </object>
    # </objects>

If the receiver is an array of hashes the root element is by default also “objects”:

    [{a: 1, b: 2}, {c: 3}].to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <objects type="array">
    #   <object>
    #     <b type="integer">2</b>
    #     <a type="integer">1</a>
    #   </object>
    #   <object>
    #     <c type="integer">3</c>
    #   </object>
    # </objects>

WARNING. If the collection is empty the root element is by default “nil-classes”. That's a gotcha, for example the root element of the list of contributors above would not be “contributors” if the collection was empty, but “nil-classes”. You may use the `:root` option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we've seen “contributor” and “object”. The option `:children` allows you to set these node names.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder via the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder:

    Contributor.limit(2).order(:rank).to_xml(skip_types: true)
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <contributors>
    #   <contributor>
    #     <id>4356</id>
    #     <name>Jeremy Kemper</name>
    #     <rank>1</rank>
    #     <url-id>jeremy-kemper</url-id>
    #   </contributor>
    #   <contributor>
    #     <id>4404</id>
    #     <name>David Heinemeier Hansson</name>
    #     <rank>2</rank>
    #     <url-id>david-heinemeier-hansson</url-id>
    #   </contributor>
    # </contributors>

NOTE: Defined in `active_support/core_ext/array/conversions.rb`.

### Wrapping¶ ↑

The method `Array.wrap` wraps its argument in an array unless it is already an array (or array-like).

Specifically:

- If the argument is `nil` an empty list is returned.

- Otherwise, if the argument responds to `to_ary` it is invoked, and if the value of `to_ary` is not `nil`, it is returned.

- Otherwise, an array with the argument as its single element is returned.

    Array.wrap(nil)       # => []
    Array.wrap([1, 2, 3]) # => [1, 2, 3]
    Array.wrap(0)         # => [0]

This method is similar in purpose to `Kernel#Array`, but there are some differences:

- If the argument responds to `to_ary` the method is invoked. `Kernel#Array` moves on to try `to_a` if the returned value is `nil`, but `Array.wrap` returns `nil` right away.

- If the returned value from `to_ary` is neither `nil` nor an `Array` object, `Kernel#Array` raises an exception, while `Array.wrap` does not, it just returns the value.

- It does not call `to_a` on the argument, though special-cases `nil` to return an empty array.

The last point is particularly worth comparing for some enumerables:

    Array.wrap(foo: :bar) # => [{:foo=>:bar}]
    Array(foo: :bar)      # => [[:foo, :bar]]

There's also a related idiom that uses the splat operator:

    [*object]

which in Ruby 1.8 returns `[nil]` for `nil`, and calls to `Array(object)` otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)

Thus, in this case the behavior is different for `nil`, and the differences with `Kernel#Array` explained above apply to the rest of `object`s.

NOTE: Defined in `active_support/core_ext/array/wrap.rb`.

### Duplicating¶ ↑

The method `Array.deep_dup` duplicates itself and all objects inside recursively with Active Support method `Object#deep_dup`. It works like `Array#map` with sending `deep_dup` method to each object inside.

    array = [1, [2, 3]]
    dup = array.deep_dup
    dup[1][2] = 4
    array[1][2] == nil   # => true

NOTE: Defined in `active_support/core_ext/object/deep_dup.rb`.

### Grouping¶ ↑

#### `in_groups_of(number, fill_with = nil)`¶ ↑

The method `in_groups_of` splits an array into consecutive groups of a certain size. It returns an array with the groups:

    [1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]

or yields them in turn if a block is passed:

    <% sample.in_groups_of(3) do |a, b, c| %>
      <tr>
        <td><%= a %></td>
        <td><%= b %></td>
        <td><%= c %></td>
      </tr>
    <% end %>

The first example shows `in_groups_of` fills the last group with as many `nil` elements as needed to have the requested size. You can change this padding value using the second optional argument:

    [1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]

And you can tell the method not to fill the last group passing `false`:

    [1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]

As a consequence `false` can't be a used as a padding value.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

#### `in_groups(number, fill_with = nil)`¶ ↑

The method `in_groups` splits an array into a certain number of groups. The method returns an array with the groups:

    %w(1 2 3 4 5 6 7).in_groups(3)
    # => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]

or yields them in turn if a block is passed:

    %w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
    ["1", "2", "3"]
    ["4", "5", nil]
    ["6", "7", nil]

The examples above show that `in_groups` fills some groups with a trailing `nil` element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

    %w(1 2 3 4 5 6 7).in_groups(3, "0")
    # => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]

And you can tell the method not to fill the smaller groups passing `false`:

    %w(1 2 3 4 5 6 7).in_groups(3, false)
    # => [["1", "2", "3"], ["4", "5"], ["6", "7"]]

As a consequence `false` can't be a used as a padding value.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

#### `split(value = nil)`¶ ↑

The method `split` divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

    (-5..5).to_a.split { |i| i.multiple_of?(4) }
    # => [[-5], [-3, -2, -1], [1, 2, 3], [5]]

Otherwise, the value received as argument, which defaults to `nil`, is the separator:

    [0, 1, -5, 1, 1, "foo", "bar"].split(1)
    # => [[0], [-5], [], ["foo", "bar"]]

TIP: Observe in the previous example that consecutive separators result in empty arrays.

NOTE: Defined in `active_support/core_ext/array/grouping.rb`.

## Extensions to `Hash`¶ ↑

### Conversions¶ ↑

#### `to_xml`¶ ↑

The method `to_xml` returns a string containing an XML representation of its receiver:

    {"foo" => 1, "bar" => 2}.to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <hash>
    #   <foo type="integer">1</foo>
    #   <bar type="integer">2</bar>
    # </hash>

To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair `key`, `value`:

- If `value` is a hash there's a recursive call with `key` as `:root`.

- If `value` is an array there's a recursive call with `key` as `:root`, and `key` singularized as `:children`.

- If `value` is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the `options` hash as first argument with `key` as `:root`, and `key` singularized as second argument. Its return value becomes a new node.

- If `value` responds to `to_xml` the method is invoked with `key` as `:root`.

- Otherwise, a node with `key` as tag is created with a string representation of `value` as text node. If `value` is `nil` an attribute “nil” set to “true” is added. Unless the option `:skip_types` exists and is true, an attribute “type” is added as well according to the following mapping:

    XML_TYPE_NAMES = {
      "Symbol"     => "symbol",
      "Fixnum"     => "integer",
      "Bignum"     => "integer",
      "BigDecimal" => "decimal",
      "Float"      => "float",
      "TrueClass"  => "boolean",
      "FalseClass" => "boolean",
      "Date"       => "date",
      "DateTime"   => "datetime",
      "Time"       => "datetime"
    }

By default the root node is “hash”, but that's configurable via the `:root` option.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder with the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder.

NOTE: Defined in `active_support/core_ext/hash/conversions.rb`.

### Merging¶ ↑

Ruby has a built-in method `Hash#merge` that merges two hashes:

    {a: 1, b: 1}.merge(a: 0, c: 2)
    # => {:a=>0, :b=>1, :c=>2}

Active Support defines a few more ways of merging hashes that may be convenient.

#### `reverse_merge` and `reverse_merge!`¶ ↑

In case of collision the key in the hash of the argument wins in `merge`. You can support option hashes with default values in a compact way with this idiom:

    options = {length: 30, omission: "..."}.merge(options)

Active Support defines `reverse_merge` in case you prefer this alternative notation:

    options = options.reverse_merge(length: 30, omission: "...")

And a bang version `reverse_merge!` that performs the merge in place:

    options.reverse_merge!(length: 30, omission: "...")

WARNING. Take into account that `reverse_merge!` may change the hash in the caller, which may or may not be a good idea.

NOTE: Defined in `active_support/core_ext/hash/reverse_merge.rb`.

#### `reverse_update`¶ ↑

The method `reverse_update` is an alias for `reverse_merge!`, explained above.

WARNING. Note that `reverse_update` has no bang.

NOTE: Defined in `active_support/core_ext/hash/reverse_merge.rb`.

#### `deep_merge` and `deep_merge!`¶ ↑

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines `Hash#deep_merge`. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:

    {a: {b: 1}}.deep_merge(a: {c: 2})
    # => {:a=>{:b=>1, :c=>2}}

The method `deep_merge!` performs a deep merge in place.

NOTE: Defined in `active_support/core_ext/hash/deep_merge.rb`.

### Deep duplicating¶ ↑

The method `Hash.deep_dup` duplicates itself and all keys and values inside recursively with Active Support method `Object#deep_dup`. It works like `Enumerator#each_with_object` with sending `deep_dup` method to each pair inside.

    hash = { a: 1, b: { c: 2, d: [3, 4] } }

    dup = hash.deep_dup
    dup[:b][:e] = 5
    dup[:b][:d] << 5

    hash[:b][:e] == nil      # => true
    hash[:b][:d] == [3, 4]   # => true

NOTE: Defined in `active_support/core_ext/object/deep_dup.rb`.

### Working with Keys¶ ↑

#### `except` and `except!`¶ ↑

The method `except` returns a hash with the keys in the argument list removed, if present:

    {a: 1, b: 2}.except(:a) # => {:b=>2}

If the receiver responds to `convert_key`, the method is called on each of the arguments. This allows `except` to play nice with hashes with indifferent access for instance:

    {a: 1}.with_indifferent_access.except(:a)  # => {}
    {a: 1}.with_indifferent_access.except("a") # => {}

There's also the bang variant `except!` that removes keys in the very receiver.

NOTE: Defined in `active_support/core_ext/hash/except.rb`.

#### `transform_keys` and `transform_keys!`¶ ↑

The method `transform_keys` accepts a block and returns a hash that has applied the block operations to each of the keys in the receiver:

    {nil => nil, 1 => 1, a: :a}.transform_keys { |key| key.to_s.upcase }
    # => {"" => nil, "A" => :a, "1" => 1}

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

    {"a" => 1, a: 2}.transform_keys { |key| key.to_s.upcase }
    # The result could either be
    # => {"A"=>2}
    # or
    # => {"A"=>1}

This method may be useful for example to build specialized conversions. For instance `stringify_keys` and `symbolize_keys` use `transform_keys` to perform their key conversions:

    def stringify_keys
      transform_keys { |key| key.to_s }
    end
    ...
    def symbolize_keys
      transform_keys { |key| key.to_sym rescue key }
    end

There's also the bang variant `transform_keys!` that applies the block operations to keys in the very receiver.

Besides that, one can use `deep_transform_keys` and `deep_transform_keys!` to perform the block operation on all the keys in the given hash and all the hashes nested into it. An example of the result is:

    {nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_transform_keys { |key| key.to_s.upcase }
    # => {""=>nil, "1"=>1, "NESTED"=>{"A"=>3, "5"=>5}}

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `stringify_keys` and `stringify_keys!`¶ ↑

The method `stringify_keys` returns a hash that has a stringified version of the keys in the receiver. It does so by sending `to_s` to them:

    {nil => nil, 1 => 1, a: :a}.stringify_keys
    # => {"" => nil, "a" => :a, "1" => 1}

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

    {"a" => 1, a: 2}.stringify_keys
    # The result could either be
    # => {"a"=>2}
    # or
    # => {"a"=>1}

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionView::Helpers::FormHelper` defines:

    def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
      options = options.stringify_keys
      options["type"] = "checkbox"
      ...
    end

The second line can safely access the “type” key, and let the user to pass either `:type` or “type”.

There's also the bang variant `stringify_keys!` that stringifies keys in the very receiver.

Besides that, one can use `deep_stringify_keys` and `deep_stringify_keys!` to stringify all the keys in the given hash and all the hashes nested into it. An example of the result is:

    {nil => nil, 1 => 1, nested: {a: 3, 5 => 5}}.deep_stringify_keys
    # => {""=>nil, "1"=>1, "nested"=>{"a"=>3, "5"=>5}}

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `symbolize_keys` and `symbolize_keys!`¶ ↑

The method `symbolize_keys` returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending `to_sym` to them:

    {nil => nil, 1 => 1, "a" => "a"}.symbolize_keys
    # => {1=>1, nil=>nil, :a=>"a"}

WARNING. Note in the previous example only one key was symbolized.

In case of key collision, one of the values will be chosen. The chosen value may not always be the same given the same hash:

    {"a" => 1, a: 2}.symbolize_keys
    # The result could either be
    # => {:a=>2}
    # or
    # => {:a=>1}

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionController::UrlRewriter` defines

    def rewrite_path(options)
      options = options.symbolize_keys
      options.update(options[:params].symbolize_keys) if options[:params]
      ...
    end

The second line can safely access the `:params` key, and let the user to pass either `:params` or “params”.

There's also the bang variant `symbolize_keys!` that symbolizes keys in the very receiver.

Besides that, one can use `deep_symbolize_keys` and `deep_symbolize_keys!` to symbolize all the keys in the given hash and all the hashes nested into it. An example of the result is:

    {nil => nil, 1 => 1, "nested" => {"a" => 3, 5 => 5}}.deep_symbolize_keys
    # => {nil=>nil, 1=>1, nested:{a:3, 5=>5}}

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `to_options` and `to_options!`¶ ↑

The methods `to_options` and `to_options!` are respectively aliases of `symbolize_keys` and `symbolize_keys!`.

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

#### `assert_valid_keys`¶ ↑

The method `assert_valid_keys` receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does `ArgumentError` is raised.

    {a: 1}.assert_valid_keys(:a)  # passes
    {a: 1}.assert_valid_keys("a") # ArgumentError

Active Record does not accept unknown options when building associations, for example. It implements that control via `assert_valid_keys`.

NOTE: Defined in `active_support/core_ext/hash/keys.rb`.

### Working with Values¶ ↑

#### `transform_values` && `transform_values!`¶ ↑

The method `transform_values` accepts a block and returns a hash that has applied the block operations to each of the values in the receiver.

    { nil => nil, 1 => 1, :x => :a }.transform_values { |value| value.to_s.upcase }
    # => {nil=>"", 1=>"1", :x=>"A"}

There's also the bang variant `transform_values!` that applies the block operations to values in the very receiver.

NOTE: Defined in `active_support/core_text/hash/transform_values.rb`.

### Slicing¶ ↑

Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

    {a: 1, b: 2, c: 3}.slice(:a, :c)
    # => {:c=>3, :a=>1}

    {a: 1, b: 2, c: 3}.slice(:b, :X)
    # => {:b=>2} # non-existing keys are ignored

If the receiver responds to `convert_key` keys are normalized:

    {a: 1, b: 2}.with_indifferent_access.slice("a")
    # => {:a=>1}

NOTE. Slicing may come in handy for sanitizing option hashes with a white list of keys.

There's also `slice!` which in addition to perform a slice in place returns what's removed:

    hash = {a: 1, b: 2}
    rest = hash.slice!(:a) # => {:b=>2}
    hash                   # => {:a=>1}

NOTE: Defined in `active_support/core_ext/hash/slice.rb`.

### Extracting¶ ↑

The method `extract!` removes and returns the key/value pairs matching the given keys.

    hash = {a: 1, b: 2}
    rest = hash.extract!(:a) # => {:a=>1}
    hash                     # => {:b=>2}

The method `extract!` returns the same subclass of Hash, that the receiver is.

    hash = {a: 1, b: 2}.with_indifferent_access
    rest = hash.extract!(:a).class
    # => ActiveSupport::HashWithIndifferentAccess

NOTE: Defined in `active_support/core_ext/hash/slice.rb`.

### Indifferent Access¶ ↑

The method `with_indifferent_access` returns an `ActiveSupport::HashWithIndifferentAccess` out of its receiver:

    {a: 1}.with_indifferent_access["a"] # => 1

NOTE: Defined in `active_support/core_ext/hash/indifferent_access.rb`.

### Compacting¶ ↑

The methods `compact` and `compact!` return a Hash without items with `nil` value.

    {a: 1, b: 2, c: nil}.compact # => {a: 1, b: 2}

NOTE: Defined in `active_support/core_ext/hash/compact.rb`.

## Extensions to `Regexp`¶ ↑

### `multiline?`¶ ↑

The method `multiline?` says whether a regexp has the `/m` flag set, that is, whether the dot matches newlines.

    %r{.}.multiline?  # => false
    %r{.}m.multiline? # => true

    Regexp.new('.').multiline?                    # => false
    Regexp.new('.', Regexp::MULTILINE).multiline? # => true

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

    def assign_route_options(segments, defaults, requirements)
      ...
      if requirement.multiline?
        raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"
      end
      ...
    end

NOTE: Defined in `active_support/core_ext/regexp.rb`.

## Extensions to `Range`¶ ↑

### `to_s`¶ ↑

Active Support extends the method `Range#to_s` so that it understands an optional format argument. As of this writing the only supported non-default format is `:db`:

    (Date.today..Date.tomorrow).to_s
    # => "2009-10-25..2009-10-26"

    (Date.today..Date.tomorrow).to_s(:db)
    # => "BETWEEN '2009-10-25' AND '2009-10-26'"

As the example depicts, the `:db` format generates a `BETWEEN` SQL clause. That is used by Active Record in its support for range values in conditions.

NOTE: Defined in `active_support/core_ext/range/conversions.rb`.

### `include?`¶ ↑

The methods `Range#include?` and `Range#===` say whether some value falls between the ends of a given instance:

    (2..3).include?(Math::E) # => true

Active Support extends these methods so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

    (1..10).include?(3..7)  # => true
    (1..10).include?(0..7)  # => false
    (1..10).include?(3..11) # => false
    (1...9).include?(3..9)  # => false

    (1..10) === (3..7)  # => true
    (1..10) === (0..7)  # => false
    (1..10) === (3..11) # => false
    (1...9) === (3..9)  # => false

NOTE: Defined in `active_support/core_ext/range/include_range.rb`.

### `overlaps?`¶ ↑

The method `Range#overlaps?` says whether any two given ranges have non-void intersection:

    (1..10).overlaps?(7..11)  # => true
    (1..10).overlaps?(0..7)   # => true
    (1..10).overlaps?(11..27) # => false

NOTE: Defined in `active_support/core_ext/range/overlaps.rb`.

## Extensions to `Date`¶ ↑

### Calculations¶ ↑

NOTE: All the following methods are defined in `active_support/core_ext/date/calculations.rb`.

INFO: The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is, `Date.new(1582, 10, 4).tomorrow` returns `Date.new(1582, 10, 15)` and so on. Please check `test/core_ext/date_ext_test.rb` in the Active Support test suite for expected behavior.

#### `Date.current`¶ ↑

Active Support defines `Date.current` to be today in the current time zone. That's like `Date.today`, except that it honors the user time zone, if defined. It also defines `Date.yesterday` and `Date.tomorrow`, and the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Date.current`.

When making Date comparisons using methods which honor the user time zone, make sure to use `Date.current` and not `Date.today`. There are cases where the user time zone might be in the future compared to the system time zone, which `Date.today` uses by default. This means `Date.today` may equal `Date.yesterday`.

#### Named dates¶ ↑

##### `prev_year`, `next_year`¶ ↑

In Ruby 1.9 `prev_year` and `next_year` return a date with the same day/month in the last or next year:

    d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
    d.prev_year              # => Fri, 08 May 2009
    d.next_year              # => Sun, 08 May 2011

If date is the 29th of February of a leap year, you obtain the 28th:

    d = Date.new(2000, 2, 29) # => Tue, 29 Feb 2000
    d.prev_year               # => Sun, 28 Feb 1999
    d.next_year               # => Wed, 28 Feb 2001

`prev_year` is aliased to `last_year`.

##### `prev_month`, `next_month`¶ ↑

In Ruby 1.9 `prev_month` and `next_month` return the date with the same day in the last or next month:

    d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
    d.prev_month             # => Thu, 08 Apr 2010
    d.next_month             # => Tue, 08 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2000, 5, 31).prev_month # => Sun, 30 Apr 2000
    Date.new(2000, 3, 31).prev_month # => Tue, 29 Feb 2000
    Date.new(2000, 5, 31).next_month # => Fri, 30 Jun 2000
    Date.new(2000, 1, 31).next_month # => Tue, 29 Feb 2000

`prev_month` is aliased to `last_month`.

##### `prev_quarter`, `next_quarter`¶ ↑

Same as `prev_month` and `next_month`. It returns the date with the same day in the previous or next quarter:

    t = Time.local(2010, 5, 8) # => Sat, 08 May 2010
    t.prev_quarter             # => Mon, 08 Feb 2010
    t.next_quarter             # => Sun, 08 Aug 2010

If such a day does not exist, the last day of the corresponding month is returned:

    Time.local(2000, 7, 31).prev_quarter  # => Sun, 30 Apr 2000
    Time.local(2000, 5, 31).prev_quarter  # => Tue, 29 Feb 2000
    Time.local(2000, 10, 31).prev_quarter # => Mon, 30 Oct 2000
    Time.local(2000, 11, 31).next_quarter # => Wed, 28 Feb 2001

`prev_quarter` is aliased to `last_quarter`.

##### `beginning_of_week`, `end_of_week`¶ ↑

The methods `beginning_of_week` and `end_of_week` return the dates for the beginning and end of the week, respectively. Weeks are assumed to start on Monday, but that can be changed passing an argument, setting thread local `Date.beginning_of_week` or `config.beginning_of_week`.

    d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
    d.beginning_of_week          # => Mon, 03 May 2010
    d.beginning_of_week(:sunday) # => Sun, 02 May 2010
    d.end_of_week                # => Sun, 09 May 2010
    d.end_of_week(:sunday)       # => Sat, 08 May 2010

`beginning_of_week` is aliased to `at_beginning_of_week` and `end_of_week` is aliased to `at_end_of_week`.

##### `monday`, `sunday`¶ ↑

The methods `monday` and `sunday` return the dates for the previous Monday and next Sunday, respectively.

    d = Date.new(2010, 5, 8)     # => Sat, 08 May 2010
    d.monday                     # => Mon, 03 May 2010
    d.sunday                     # => Sun, 09 May 2010

    d = Date.new(2012, 9, 10)    # => Mon, 10 Sep 2012
    d.monday                     # => Mon, 10 Sep 2012

    d = Date.new(2012, 9, 16)    # => Sun, 16 Sep 2012
    d.sunday                     # => Sun, 16 Sep 2012

##### `prev_week`, `next_week`¶ ↑

The method `next_week` receives a symbol with a day name in English (default is the thread local `Date.beginning_of_week`, or `config.beginning_of_week`, or `:monday`) and it returns the date corresponding to that day.

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.next_week              # => Mon, 10 May 2010
    d.next_week(:saturday)   # => Sat, 15 May 2010

The method `prev_week` is analogous:

    d.prev_week              # => Mon, 26 Apr 2010
    d.prev_week(:saturday)   # => Sat, 01 May 2010
    d.prev_week(:friday)     # => Fri, 30 Apr 2010

`prev_week` is aliased to `last_week`.

Both `next_week` and `prev_week` work as expected when `Date.beginning_of_week` or `config.beginning_of_week` are set.

##### `beginning_of_month`, `end_of_month`¶ ↑

The methods `beginning_of_month` and `end_of_month` return the dates for the beginning and end of the month:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_month     # => Sat, 01 May 2010
    d.end_of_month           # => Mon, 31 May 2010

`beginning_of_month` is aliased to `at_beginning_of_month`, and `end_of_month` is aliased to `at_end_of_month`.

##### `beginning_of_quarter`, `end_of_quarter`¶ ↑

The methods `beginning_of_quarter` and `end_of_quarter` return the dates for the beginning and end of the quarter of the receiver's calendar year:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_quarter   # => Thu, 01 Apr 2010
    d.end_of_quarter         # => Wed, 30 Jun 2010

`beginning_of_quarter` is aliased to `at_beginning_of_quarter`, and `end_of_quarter` is aliased to `at_end_of_quarter`.

##### `beginning_of_year`, `end_of_year`¶ ↑

The methods `beginning_of_year` and `end_of_year` return the dates for the beginning and end of the year:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_year      # => Fri, 01 Jan 2010
    d.end_of_year            # => Fri, 31 Dec 2010

`beginning_of_year` is aliased to `at_beginning_of_year`, and `end_of_year` is aliased to `at_end_of_year`.

#### Other Date Computations¶ ↑

##### `years_ago`, `years_since`¶ ↑

The method `years_ago` receives a number of years and returns the same date those many years ago:

    date = Date.new(2010, 6, 7)
    date.years_ago(10) # => Wed, 07 Jun 2000

`years_since` moves forward in time:

    date = Date.new(2010, 6, 7)
    date.years_since(10) # => Sun, 07 Jun 2020

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2012, 2, 29).years_ago(3)     # => Sat, 28 Feb 2009
    Date.new(2012, 2, 29).years_since(3)   # => Sat, 28 Feb 2015

##### `months_ago`, `months_since`¶ ↑

The methods `months_ago` and `months_since` work analogously for months:

    Date.new(2010, 4, 30).months_ago(2)   # => Sun, 28 Feb 2010
    Date.new(2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2010, 4, 30).months_ago(2)    # => Sun, 28 Feb 2010
    Date.new(2009, 12, 31).months_since(2) # => Sun, 28 Feb 2010

##### `weeks_ago`¶ ↑

The method `weeks_ago` works analogously for weeks:

    Date.new(2010, 5, 24).weeks_ago(1)    # => Mon, 17 May 2010
    Date.new(2010, 5, 24).weeks_ago(2)    # => Mon, 10 May 2010

##### `advance`¶ ↑

The most generic way to jump to other days is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, and returns a date advanced as much as the present keys indicate:

    date = Date.new(2010, 6, 6)
    date.advance(years: 1, weeks: 2)  # => Mon, 20 Jun 2011
    date.advance(months: 2, days: -2) # => Wed, 04 Aug 2010

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method `advance` advances first one month, and then one day, the result is:

    Date.new(2010, 2, 28).advance(months: 1, days: 1)
    # => Sun, 29 Mar 2010

While if it did it the other way around the result would be different:

    Date.new(2010, 2, 28).advance(days: 1).advance(months: 1)
    # => Thu, 01 Apr 2010

#### Changing Components¶ ↑

The method `change` allows you to get a new date which is the same as the receiver except for the given year, month, or day:

    Date.new(2010, 12, 23).change(year: 2011, month: 11)
    # => Wed, 23 Nov 2011

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

    Date.new(2010, 1, 31).change(month: 2)
    # => ArgumentError: invalid date

#### Durations¶ ↑

Durations can be added to and subtracted from dates:

    d = Date.current
    # => Mon, 09 Aug 2010
    d + 1.year
    # => Tue, 09 Aug 2011
    d - 3.hours
    # => Sun, 08 Aug 2010 21:00:00 UTC +00:00

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    Date.new(1582, 10, 4) + 1.day
    # => Fri, 15 Oct 1582

#### Timestamps¶ ↑

INFO: The following methods return a `Time` object if possible, otherwise a `DateTime`. If set, they honor the user time zone.

##### `beginning_of_day`, `end_of_day`¶ ↑

The method `beginning_of_day` returns a timestamp at the beginning of the day (00:00:00):

    date = Date.new(2010, 6, 7)
    date.beginning_of_day # => Mon Jun 07 00:00:00 +0200 2010

The method `end_of_day` returns a timestamp at the end of the day (23:59:59):

    date = Date.new(2010, 6, 7)
    date.end_of_day # => Mon Jun 07 23:59:59 +0200 2010

`beginning_of_day` is aliased to `at_beginning_of_day`, `midnight`, `at_midnight`.

##### `beginning_of_hour`, `end_of_hour`¶ ↑

The method `beginning_of_hour` returns a timestamp at the beginning of the hour (hh:00:00):

    date = DateTime.new(2010, 6, 7, 19, 55, 25)
    date.beginning_of_hour # => Mon Jun 07 19:00:00 +0200 2010

The method `end_of_hour` returns a timestamp at the end of the hour (hh:59:59):

    date = DateTime.new(2010, 6, 7, 19, 55, 25)
    date.end_of_hour # => Mon Jun 07 19:59:59 +0200 2010

`beginning_of_hour` is aliased to `at_beginning_of_hour`.

##### `beginning_of_minute`, `end_of_minute`¶ ↑

The method `beginning_of_minute` returns a timestamp at the beginning of the minute (hh:mm:00):

    date = DateTime.new(2010, 6, 7, 19, 55, 25)
    date.beginning_of_minute # => Mon Jun 07 19:55:00 +0200 2010

The method `end_of_minute` returns a timestamp at the end of the minute (hh:mm:59):

    date = DateTime.new(2010, 6, 7, 19, 55, 25)
    date.end_of_minute # => Mon Jun 07 19:55:59 +0200 2010

`beginning_of_minute` is aliased to `at_beginning_of_minute`.

INFO: `beginning_of_hour`, `end_of_hour`, `beginning_of_minute` and `end_of_minute` are implemented for `Time` and `DateTime` but not `Date` as it does not make sense to request the beginning or end of an hour or minute on a `Date` instance.

##### `ago`, `since`¶ ↑

The method `ago` receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

    date = Date.current # => Fri, 11 Jun 2010
    date.ago(1)         # => Thu, 10 Jun 2010 23:59:59 EDT -04:00

Similarly, `since` moves forward:

    date = Date.current # => Fri, 11 Jun 2010
    date.since(1)       # => Fri, 11 Jun 2010 00:00:01 EDT -04:00

#### Other Time Computations¶ ↑

### Conversions¶ ↑

## Extensions to `DateTime`¶ ↑

WARNING: `DateTime` is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example `seconds_since_midnight` might not return the real amount in such a day.

### Calculations¶ ↑

NOTE: All the following methods are defined in `active_support/core_ext/date_time/calculations.rb`.

The class `DateTime` is a subclass of `Date` so by loading `active_support/core_ext/date/calculations.rb` you inherit these methods and their aliases, except that they will always return datetimes:

    yesterday
    tomorrow
    beginning_of_week (at_beginning_of_week)
    end_of_week (at_end_of_week)
    monday
    sunday
    weeks_ago
    prev_week (last_week)
    next_week
    months_ago
    months_since
    beginning_of_month (at_beginning_of_month)
    end_of_month (at_end_of_month)
    prev_month (last_month)
    next_month
    beginning_of_quarter (at_beginning_of_quarter)
    end_of_quarter (at_end_of_quarter)
    beginning_of_year (at_beginning_of_year)
    end_of_year (at_end_of_year)
    years_ago
    years_since
    prev_year (last_year)
    next_year

The following methods are reimplemented so you do not need to load `active_support/core_ext/date/calculations.rb` for these ones:

    beginning_of_day (midnight, at_midnight, at_beginning_of_day)
    end_of_day
    ago
    since (in)

On the other hand, `advance` and `change` are also defined and support more options, they are documented below.

The following methods are only implemented in `active_support/core_ext/date_time/calculations.rb` as they only make sense when used with a `DateTime` instance:

    beginning_of_hour (at_beginning_of_hour)
    end_of_hour

#### Named Datetimes¶ ↑

##### `DateTime.current`¶ ↑

Active Support defines `DateTime.current` to be like `Time.now.to_datetime`, except that it honors the user time zone, if defined. It also defines `DateTime.yesterday` and `DateTime.tomorrow`, and the instance predicates `past?`, and `future?` relative to `DateTime.current`.

#### Other Extensions¶ ↑

##### `seconds_since_midnight`¶ ↑

The method `seconds_since_midnight` returns the number of seconds since midnight:

    now = DateTime.current     # => Mon, 07 Jun 2010 20:26:36 +0000
    now.seconds_since_midnight # => 73596

##### `utc`¶ ↑

The method `utc` gives you the same datetime in the receiver expressed in UTC.

    now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
    now.utc                # => Mon, 07 Jun 2010 23:27:52 +0000

This method is also aliased as `getutc`.

##### `utc?`¶ ↑

The predicate `utc?` says whether the receiver has UTC as its time zone:

    now = DateTime.now # => Mon, 07 Jun 2010 19:30:47 -0400
    now.utc?           # => false
    now.utc.utc?       # => true

##### `advance`¶ ↑

The most generic way to jump to another datetime is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, `:hours`, `:minutes`, and `:seconds`, and returns a datetime advanced as much as the present keys indicate.

    d = DateTime.current
    # => Thu, 05 Aug 2010 11:33:31 +0000
    d.advance(years: 1, months: 1, days: 1, hours: 1, minutes: 1, seconds: 1)
    # => Tue, 06 Sep 2011 12:34:32 +0000

This method first computes the destination date passing `:years`, `:months`, `:weeks`, and `:days` to `Date#advance` documented above. After that, it adjusts the time calling `since` with the number of seconds to advance. This order is relevant, a different ordering would give different datetimes in some edge-cases. The example in `Date#advance` applies, and we can extend it to show order relevance related to the time bits.

If we first move the date bits (that have also a relative order of processing, as documented before), and then the time bits we get for example the following computation:

    d = DateTime.new(2010, 2, 28, 23, 59, 59)
    # => Sun, 28 Feb 2010 23:59:59 +0000
    d.advance(months: 1, seconds: 1)
    # => Mon, 29 Mar 2010 00:00:00 +0000

but if we computed them the other way around, the result would be different:

    d.advance(seconds: 1).advance(months: 1)
    # => Thu, 01 Apr 2010 00:00:00 +0000

WARNING: Since `DateTime` is not DST-aware you can end up in a non-existing point in time with no warning or error telling you so.

#### Changing Components¶ ↑

The method `change` allows you to get a new datetime which is the same as the receiver except for the given options, which may include `:year`, `:month`, `:day`, `:hour`, `:min`, `:sec`, `:offset`, `:start`:

    now = DateTime.current
    # => Tue, 08 Jun 2010 01:56:22 +0000
    now.change(year: 2011, offset: Rational(-6, 24))
    # => Wed, 08 Jun 2011 01:56:22 -0600

If hours are zeroed, then minutes and seconds are too (unless they have given values):

    now.change(hour: 0)
    # => Tue, 08 Jun 2010 00:00:00 +0000

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

    now.change(min: 0)
    # => Tue, 08 Jun 2010 01:00:00 +0000

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

    DateTime.current.change(month: 2, day: 30)
    # => ArgumentError: invalid date

#### Durations¶ ↑

Durations can be added to and subtracted from datetimes:

    now = DateTime.current
    # => Mon, 09 Aug 2010 23:15:17 +0000
    now + 1.year
    # => Tue, 09 Aug 2011 23:15:17 +0000
    now - 1.week
    # => Mon, 02 Aug 2010 23:15:17 +0000

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    DateTime.new(1582, 10, 4, 23) + 1.hour
    # => Fri, 15 Oct 1582 00:00:00 +0000

## Extensions to `Time`¶ ↑

### Calculations¶ ↑

NOTE: All the following methods are defined in `active_support/core_ext/time/calculations.rb`.

Active Support adds to `Time` many of the methods available for `DateTime`:

    past?
    today?
    future?
    yesterday
    tomorrow
    seconds_since_midnight
    change
    advance
    ago
    since (in)
    beginning_of_day (midnight, at_midnight, at_beginning_of_day)
    end_of_day
    beginning_of_hour (at_beginning_of_hour)
    end_of_hour
    beginning_of_week (at_beginning_of_week)
    end_of_week (at_end_of_week)
    monday
    sunday
    weeks_ago
    prev_week (last_week)
    next_week
    months_ago
    months_since
    beginning_of_month (at_beginning_of_month)
    end_of_month (at_end_of_month)
    prev_month (last_month)
    next_month
    beginning_of_quarter (at_beginning_of_quarter)
    end_of_quarter (at_end_of_quarter)
    beginning_of_year (at_beginning_of_year)
    end_of_year (at_end_of_year)
    years_ago
    years_since
    prev_year (last_year)
    next_year

They are analogous. Please refer to their documentation above and take into account the following differences:

- `change` accepts an additional `:usec` option.

- `Time` understands DST, so you get correct DST calculations as in

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>

    # In Barcelona, 2010/03/28 02:00 +0100 becomes 2010/03/28 03:00 +0200 due to DST.
    t = Time.local(2010, 3, 28, 1, 59, 59)
    # => Sun Mar 28 01:59:59 +0100 2010
    t.advance(seconds: 1)
    # => Sun Mar 28 03:00:00 +0200 2010

- If `since` or `ago` jump to a time that can't be expressed with `Time` a `DateTime` object is returned instead.

#### `Time.current`¶ ↑

Active Support defines `Time.current` to be today in the current time zone. That's like `Time.now`, except that it honors the user time zone, if defined. It also defines the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Time.current`.

When making Time comparisons using methods which honor the user time zone, make sure to use `Time.current` instead of `Time.now`. There are cases where the user time zone might be in the future compared to the system time zone, which `Time.now` uses by default. This means `Time.now.to_date` may equal `Date.yesterday`.

#### `all_day`, `all_week`, `all_month`, `all_quarter` and `all_year`¶ ↑

The method `all_day` returns a range representing the whole day of the current time.

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now.all_day
    # => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Mon, 09 Aug 2010 23:59:59 UTC +00:00

Analogously, `all_week`, `all_month`, `all_quarter` and `all_year` all serve the purpose of generating time ranges.

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now.all_week
    # => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Sun, 15 Aug 2010 23:59:59 UTC +00:00
    now.all_week(:sunday)
    # => Sun, 16 Sep 2012 00:00:00 UTC +00:00..Sat, 22 Sep 2012 23:59:59 UTC +00:00
    now.all_month
    # => Sat, 01 Aug 2010 00:00:00 UTC +00:00..Tue, 31 Aug 2010 23:59:59 UTC +00:00
    now.all_quarter
    # => Thu, 01 Jul 2010 00:00:00 UTC +00:00..Thu, 30 Sep 2010 23:59:59 UTC +00:00
    now.all_year
    # => Fri, 01 Jan 2010 00:00:00 UTC +00:00..Fri, 31 Dec 2010 23:59:59 UTC +00:00

### Time Constructors¶ ↑

Active Support defines `Time.current` to be `Time.zone.now` if there's a user time zone defined, with fallback to `Time.now`:

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
    Time.current
    # => Fri, 06 Aug 2010 17:11:58 CEST +02:00

Analogously to `DateTime`, the predicates `past?`, and `future?` are relative to `Time.current`.

If the time to be constructed lies beyond the range supported by `Time` in the runtime platform, usecs are discarded and a `DateTime` object is returned instead.

#### Durations¶ ↑

Durations can be added to and subtracted from time objects:

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now + 1.year
    #  => Tue, 09 Aug 2011 23:21:11 UTC +00:00
    now - 1.week
    # => Mon, 02 Aug 2010 23:21:11 UTC +00:00

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    Time.utc(1582, 10, 3) + 5.days
    # => Mon Oct 18 00:00:00 UTC 1582

## Extensions to `File`¶ ↑

### `atomic_write`¶ ↑

With the class method `File.atomic_write` you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done `atomic_write` closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like `all.css`:

    File.atomic_write(joined_asset_path) do |cache|
      cache.write(join_asset_file_contents(asset_paths))
    end

To accomplish this `atomic_write` creates a temporary file. That's the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists `atomic_write` overwrites it and keeps owners and permissions. However there are a few cases where `atomic_write` cannot change the file ownership or permissions, this error is caught and skipped over trusting in the user/filesystem to ensure the file is accessible to the processes that need it.

NOTE. Due to the chmod operation `atomic_write` performs, if the target file has an ACL set on it this ACL will be recalculated/modified.

WARNING. Note you can't append with `atomic_write`.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

NOTE: Defined in `active_support/core_ext/file/atomic.rb`.

## Extensions to `Marshal`¶ ↑

### `load`¶ ↑

Active Support adds constant autoloading support to `load`.

For example, the file cache store deserializes this way:

    File.open(file_name) { |f| Marshal.load(f) }

If the cached data refers to a constant that is unknown at that point, the autoloading mechanism is triggered and if it succeeds the deserialization is retried transparently.

WARNING. If the argument is an `IO` it needs to respond to `rewind` to be able to retry. Regular files respond to `rewind`.

NOTE: Defined in `active_support/core_ext/marshal.rb`.

## Extensions to `Logger`¶ ↑

### `around_[level]`¶ ↑

Takes two arguments, a `before_message` and `after_message` and calls the current level method on the `Logger` instance, passing in the `before_message`, then the specified message, then the `after_message`:

    logger = Logger.new("log/development.log")
    logger.around_info("before", "after") { |logger| logger.info("during") }

### `silence`¶ ↑

Silences every log level lesser to the specified one for the duration of the given block. Log level orders are: debug, info, error and fatal.

    logger = Logger.new("log/development.log")
    logger.silence(Logger::INFO) do
      logger.debug("In space, no one can hear you scream.")
      logger.info("Scream all you want, small mailman!")
    end

### `datetime_format=`¶ ↑

Modifies the datetime format output by the formatter class associated with this logger. If the formatter class does not have a `datetime_format` method then this is ignored.

    class Logger::FormatWithTime < Logger::Formatter
      cattr_accessor(:datetime_format) { "%Y%m%d%H%m%S" }

      def self.call(severity, timestamp, progname, msg)
        "#{timestamp.strftime(datetime_format)} -- #{String === msg ? msg : msg.inspect}\n"
      end
    end

    logger = Logger.new("log/development.log")
    logger.formatter = Logger::FormatWithTime
    logger.info("<- is the current time")

NOTE: Defined in `active_support/core_ext/logger.rb`.

## Extensions to `NameError`¶ ↑

Active Support adds `missing_name?` to `NameError`, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

TIP: A symbol can represent a fully-qualified constant name as in `:"ActiveRecord::Base"`, so the behavior for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of `ArticlesController` is called Rails tries optimistically to use `ArticlesHelper`. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that `articles_helper.rb` raises a `NameError` due to an actual unknown constant. That should be reraised. The method `missing_name?` provides a way to distinguish both cases:

    def default_helper_module!
      module_name = name.sub(/Controller$/, '')
      module_path = module_name.underscore
      helper module_path
    rescue MissingSourceFile => e
      raise e unless e.is_missing? "helpers/#{module_path}_helper"
    rescue NameError => e
      raise e unless e.missing_name? "#{module_name}Helper"
    end

NOTE: Defined in `active_support/core_ext/name_error.rb`.

## Extensions to `LoadError`¶ ↑

Active Support adds `is_missing?` to `LoadError`, and also assigns that class to the constant `MissingSourceFile` for backwards compatibility.

Given a path name `is_missing?` tests whether the exception was raised due to that particular file (except perhaps for the “.rb” extension).

For example, when an action of `ArticlesController` is called Rails tries to load `articles_helper.rb`, but that file may not exist. That's fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method `is_missing?` provides a way to distinguish both cases:

    def default_helper_module!
      module_name = name.sub(/Controller$/, '')
      module_path = module_name.underscore
      helper module_path
    rescue MissingSourceFile => e
      raise e unless e.is_missing? "helpers/#{module_path}_helper"
    rescue NameError => e
      raise e unless e.missing_name? "#{module_name}Helper"
    end

NOTE: Defined in `active_support/core_ext/load_error.rb`.