# Autoloading and Reloading Constants¶ ↑

This guide documents how constant autoloading and reloading works.

After reading this guide, you will know:

- Key aspects of Ruby constants

- What is `autoload_paths`

- How constant autoloading works

- What is `require_dependency`

- How constant reloading works

- Solutions to common autoloading gotchas

## Introduction¶ ↑

Ruby on Rails allows applications to be written as if their code was preloaded.

In a normal Ruby program classes need to load their dependencies:

    require 'application_controller'
    require 'post'

    class PostsController < ApplicationController
      def index
        @posts = Post.all
      end
    end

Our Rubyist instinct quickly sees some redundancy in there: If classes were defined in files matching their name, couldn't their loading be automated somehow? We could save scanning the file for dependencies, which is brittle.

Moreover, `Kernel#require` loads files once, but development is much more smooth if code gets refreshed when it changes without restarting the server. It would be nice to be able to use `Kernel#load` in development, and `Kernel#require` in production.

Indeed, those features are provided by Ruby on Rails, where we just write

    class PostsController < ApplicationController
      def index
        @posts = Post.all
      end
    end

This guide documents how that works.

## Constants Refresher¶ ↑

While constants are trivial in most programming languages, they are a rich topic in Ruby.

It is beyond the scope of this guide to document Ruby constants, but we are nevertheless going to highlight a few key topics. Truly grasping the following sections is instrumental to understanding constant autoloading and reloading.

### Nesting¶ ↑

Class and module definitions can be nested to create namespaces:

    module XML
      class SAXParser
        # (1)
      end
    end

The nesting at any given place is the collection of enclosing nested class and module objects outwards. The nesting at any given place can be inspected with `Module.nesting`. For example, in the previous example, the nesting at (1) is

    [XML::SAXParser, XML]

It is important to understand that the nesting is composed of class and module objects, it has nothing to do with the constants used to access them, and is also unrelated to their names.

For instance, while this definition is similar to the previous one:

    class XML::SAXParser
      # (2)
    end

the nesting in (2) is different:

    [XML::SAXParser]

`XML` does not belong to it.

We can see in this example that the name of a class or module that belongs to a certain nesting does not necessarily correlate with the namespaces at the spot.

Even more, they are totally independent, take for instance

    module X
      module Y
      end
    end

    module A
      module B
      end
    end

    module X::Y
      module A::B
        # (3)
      end
    end

The nesting in (3) consists of two module objects:

    [A::B, X::Y]

So, it not only doesn't end in `A`, which does not even belong to the nesting, but it also contains `X::Y`, which is independent from `A::B`.

The nesting is an internal stack maintained by the interpreter, and it gets modified according to these rules:

- The class object following a `class` keyword gets pushed when its body is executed, and popped after it.

- The module object following a `module` keyword gets pushed when its body is executed, and popped after it.

- A singleton class opened with `class << object` gets pushed, and popped later.

- When `instance_eval` is called using a string argument, the singleton class of the receiver is pushed to the nesting of the eval'ed code. When `class_eval` or `module_eval` is called using a string argument, the receiver is pushed to the nesting of the eval'ed code.

- The nesting at the top-level of code interpreted by `Kernel#load` is empty unless the `load` call receives a true value as second argument, in which case a newly created anonymous module is pushed by Ruby.

It is interesting to observe that blocks do not modify the stack. In particular the blocks that may be passed to `Class.new` and `Module.new` do not get the class or module being defined pushed to their nesting. That's one of the differences between defining classes and modules in one way or another.

### Class and Module Definitions are Constant Assignments¶ ↑

Let's suppose the following snippet creates a class (rather than reopening it):

    class C
    end

Ruby creates a constant `C` in `Object` and stores in that constant a class object. The name of the class instance is “C”, a string, named after the constant.

That is,

    class Project < ActiveRecord::Base
    end

performs a constant assignment equivalent to

    Project = Class.new(ActiveRecord::Base)

including setting the name of the class as a side-effect:

    Project.name # => "Project"

Constant assignment has a special rule to make that happen: if the object being assigned is an anonymous class or module, Ruby sets the object's name to the name of the constant.

INFO. From then on, what happens to the constant and the instance does not matter. For example, the constant could be deleted, the class object could be assigned to a different constant, be stored in no constant anymore, etc. Once the name is set, it doesn't change.

Similarly, module creation using the `module` keyword as in

    module Admin
    end

performs a constant assignment equivalent to

    Admin = Module.new

including setting the name as a side-effect:

    Admin.name # => "Admin"

WARNING. The execution context of a block passed to `Class.new` or `Module.new` is not entirely equivalent to the one of the body of the definitions using the `class` and `module` keywords. But both idioms result in the same constant assignment.

Thus, when one informally says “the `String` class”, that really means: the class object stored in the constant called “String” in the class object stored in the `Object` constant. `String` is otherwise an ordinary Ruby constant and everything related to constants such as resolution algorithms applies to it.

Likewise, in the controller

    class PostsController < ApplicationController
      def index
        @posts = Post.all
      end
    end

`Post` is not syntax for a class. Rather, `Post` is a regular Ruby constant. If all is good, the constant is evaluated to an object that responds to `all`.

That is why we talk about constant autoloading, Rails has the ability to load constants on the fly.

### Constants are Stored in Modules¶ ↑

Constants belong to modules in a very literal sense. Classes and modules have a constant table; think of it as a hash table.

Let's analyze an example to really understand what that means. While common abuses of language like “the `String` class” are convenient, the exposition is going to be precise here for didactic purposes.

Let's consider the following module definition:

    module Colors
      RED = '0xff0000'
    end

First, when the `module` keyword is processed, the interpreter creates a new entry in the constant table of the class object stored in the `Object` constant. Said entry associates the name “Colors” to a newly created module object. Furthermore, the interpreter sets the name of the new module object to be the string “Colors”.

Later, when the body of the module definition is interpreted, a new entry is created in the constant table of the module object stored in the `Colors` constant. That entry maps the name “RED” to the string “0xff0000”.

In particular, `Colors::RED` is totally unrelated to any other `RED` constant that may live in any other class or module object. If there were any, they would have separate entries in their respective constant tables.

Pay special attention in the previous paragraphs to the distinction between class and module objects, constant names, and value objects associated to them in constant tables.

### Resolution Algorithms¶ ↑

#### Resolution Algorithm for Relative Constants¶ ↑

At any given place in the code, let's define cref to be the first element of the nesting if it is not empty, or `Object` otherwise.

Without getting too much into the details, the resolution algorithm for relative constant references goes like this:

1. If the nesting is not empty the constant is looked up in its elements and in order. The ancestors of those elements are ignored.

2. If not found, then the algorithm walks up the ancestor chain of the cref.

3. If not found and the cref is a module, the constant is looked up in `Object`.

4. If not found, `const_missing` is invoked on the cref. The default implementation of `const_missing` raises `NameError`, but it can be overridden.

Rails autoloading does not emulate this algorithm, but its starting point is the name of the constant to be autoloaded, and the cref. See more in Relative References.

#### Resolution Algorithm for Qualified Constants¶ ↑

Qualified constants look like this:

    Billing::Invoice

`Billing::Invoice` is composed of two constants: `Billing` is relative and is resolved using the algorithm of the previous section.

INFO. Leading colons would make the first segment absolute rather than relative: `::Billing::Invoice`. That would force `Billing` to be looked up only as a top-level constant.

`Invoice` on the other hand is qualified by `Billing` and we are going to see its resolution next. Let's define parent to be that qualifying class or module object, that is, `Billing` in the example above. The algorithm for qualified constants goes like this:

1. The constant is looked up in the parent and its ancestors.

2. If the lookup fails, `const_missing` is invoked in the parent. The default implementation of `const_missing` raises `NameError`, but it can be overridden.

As you see, this algorithm is simpler than the one for relative constants. In particular, the nesting plays no role here, and modules are not special-cased, if neither they nor their ancestors have the constants, `Object` is not checked.

Rails autoloading does not emulate this algorithm, but its starting point is the name of the constant to be autoloaded, and the parent. See more in Qualified References.

## Vocabulary¶ ↑

### Parent Namespaces¶ ↑

Given a string with a constant path we define its parent namespace to be the string that results from removing its rightmost segment.

For example, the parent namespace of the string “A::B::C” is the string “A::B”, the parent namespace of “A::B” is “A”, and the parent namespace of “A” is “”.

The interpretation of a parent namespace when thinking about classes and modules is tricky though. Let's consider a module M named “A::B”:

- The parent namespace, “A”, may not reflect nesting at a given spot.

- The constant `A` may no longer exist, some code could have removed it from `Object`.

- If `A` exists, the class or module that was originally in `A` may not be there anymore. For example, if after a constant removal there was another constant assignment there would generally be a different object in there.

- In such case, it could even happen that the reassigned `A` held a new class or module called also “A”!

- In the previous scenarios M would no longer be reachable through `A::B` but the module object itself could still be alive somewhere and its name would still be “A::B”.

The idea of a parent namespace is at the core of the autoloading algorithms and helps explain and understand their motivation intuitively, but as you see that metaphor leaks easily. Given an edge case to reason about, take always into account that by “parent namespace” the guide means exactly that specific string derivation.

### Loading Mechanism¶ ↑

Rails autoloads files with `Kernel#load` when `config.cache_classes` is false, the default in development mode, and with `Kernel#require` otherwise, the default in production mode.

`Kernel#load` allows Rails to execute files more than once if constant reloading is enabled.

This guide uses the word “load” freely to mean a given file is interpreted, but the actual mechanism can be `Kernel#load` or `Kernel#require` depending on that flag.

## Autoloading Availability¶ ↑

Rails is always able to autoload provided its environment is in place. For example the `runner` command autoloads:

    $ bin/rails runner 'p User.column_names'
    ["id", "email", "created_at", "updated_at"]

The console autoloads, the test suite autoloads, and of course the application autoloads.

By default, Rails eager loads the application files when it boots in production mode, so most of the autoloading going on in development does not happen. But autoloading may still be triggered during eager loading.

For example, given

    class BeachHouse < House
    end

if `House` is still unknown when `app/models/beach_house.rb` is being eager loaded, Rails autoloads it.

## autoload_paths¶ ↑

As you probably know, when `require` gets a relative file name:

    require 'erb'

Ruby looks for the file in the directories listed in `$LOAD_PATH`. That is, Ruby iterates over all its directories and for each one of them checks whether they have a file called “erb.rb”, or “erb.so”, or “erb.o”, or “erb.dll”. If it finds any of them, the interpreter loads it and ends the search. Otherwise, it tries again in the next directory of the list. If the list gets exhausted, `LoadError` is raised.

We are going to cover how constant autoloading works in more detail later, but the idea is that when a constant like `Post` is hit and missing, if there's a `post.rb` file for example in `app/models` Rails is going to find it, evaluate it, and have `Post` defined as a side-effect.

Alright, Rails has a collection of directories similar to `$LOAD_PATH` in which to look up `post.rb`. That collection is called `autoload_paths` and by default it contains:

- All subdirectories of `app` in the application and engines. For example, `app/controllers`. They do not need to be the default ones, any custom directories like `app/workers` belong automatically to `autoload_paths`.

- Second level directories `app/{controllers,models}/concerns` in the application and engines.

- The directory `test/mailers/previews`.

Also, this collection is configurable via `config.autoload_paths`. For example, `lib` was in the list years ago, but no longer is. An application can opt-in by adding this to `config/application.rb`:

    config.autoload_paths << "#{Rails.root}/lib"

`config.autoload_paths` is accessible from environment-specific configuration files, but any changes made to it outside `config/application.rb` don't have an effect.

The value of `autoload_paths` can be inspected. In a just generated application it is (edited):

    $ bin/rails r 'puts ActiveSupport::Dependencies.autoload_paths'
    .../app/assets
    .../app/controllers
    .../app/helpers
    .../app/mailers
    .../app/models
    .../app/controllers/concerns
    .../app/models/concerns
    .../test/mailers/previews

INFO. `autoload_paths` is computed and cached during the initialization process. The application needs to be restarted to reflect any changes in the directory structure.

## Autoloading Algorithms¶ ↑

### Relative References¶ ↑

A relative constant reference may appear in several places, for example, in

    class PostsController < ApplicationController
      def index
        @posts = Post.all
      end
    end

all three constant references are relative.

#### Constants after the `class` and `module` Keywords¶ ↑

Ruby performs a lookup for the constant that follows a `class` or `module` keyword because it needs to know if the class or module is going to be created or reopened.

If the constant is not defined at that point it is not considered to be a missing constant, autoloading is not triggered.

So, in the previous example, if `PostsController` is not defined when the file is interpreted Rails autoloading is not going to be triggered, Ruby will just define the controller.

#### Top-Level Constants¶ ↑

On the contrary, if `ApplicationController` is unknown, the constant is considered missing and an autoload is going to be attempted by Rails.

In order to load `ApplicationController`, Rails iterates over `autoload_paths`. First checks if `app/assets/application_controller.rb` exists. If it does not, which is normally the case, it continues and finds `app/controllers/application_controller.rb`.

If the file defines the constant `ApplicationController` all is fine, otherwise `LoadError` is raised:

    unable to autoload constant ApplicationController, expected
    <full path to application_controller.rb> to define it (LoadError)

INFO. Rails does not require the value of autoloaded constants to be a class or module object. For example, if the file `app/models/max_clients.rb` defines `MAX_CLIENTS = 100` autoloading `MAX_CLIENTS` works just fine.

#### Namespaces¶ ↑

Autoloading `ApplicationController` looks directly under the directories of `autoload_paths` because the nesting in that spot is empty. The situation of `Post` is different, the nesting in that line is `[PostsController]` and support for namespaces comes into play.

The basic idea is that given

    module Admin
      class BaseController < ApplicationController
        @@all_roles = Role.all
      end
    end

to autoload `Role` we are going to check if it is defined in the current or parent namespaces, one at a time. So, conceptually we want to try to autoload any of

    Admin::BaseController::Role
    Admin::Role
    Role

in that order. That's the idea. To do so, Rails looks in `autoload_paths` respectively for file names like these:

    admin/base_controller/role.rb
    admin/role.rb
    role.rb

modulus some additional directory lookups we are going to cover soon.

INFO. `'Constant::Name'.underscore` gives the relative path without extension of the file name where `Constant::Name` is expected to be defined.

Let's see how Rails autoloads the `Post` constant in the `PostsController` above assuming the application has a `Post` model defined in `app/models/post.rb`.

First it checks for `posts_controller/post.rb` in `autoload_paths`:

    app/assets/posts_controller/post.rb
    app/controllers/posts_controller/post.rb
    app/helpers/posts_controller/post.rb
    ...
    test/mailers/previews/posts_controller/post.rb

Since the lookup is exhausted without success, a similar search for a directory is performed, we are going to see why in the next section:

    app/assets/posts_controller/post
    app/controllers/posts_controller/post
    app/helpers/posts_controller/post
    ...
    test/mailers/previews/posts_controller/post

If all those attempts fail, then Rails starts the lookup again in the parent namespace. In this case only the top-level remains:

    app/assets/post.rb
    app/controllers/post.rb
    app/helpers/post.rb
    app/mailers/post.rb
    app/models/post.rb

A matching file is found in `app/models/post.rb`. The lookup stops there and the file is loaded. If the file actually defines `Post` all is fine, otherwise `LoadError` is raised.

### Qualified References¶ ↑

When a qualified constant is missing Rails does not look for it in the parent namespaces. But there is a caveat: When a constant is missing, Rails is unable to tell if the trigger was a relative reference or a qualified one.

For example, consider

    module Admin
      User
    end

and

    Admin::User

If `User` is missing, in either case all Rails knows is that a constant called “User” was missing in a module called “Admin”.

If there is a top-level `User` Ruby would resolve it in the former example, but wouldn't in the latter. In general, Rails does not emulate the Ruby constant resolution algorithms, but in this case it tries using the following heuristic:

> If none of the parent namespaces of the class or module has the missing constant then Rails assumes the reference is relative. Otherwise qualified.

For example, if this code triggers autoloading

    Admin::User

and the `User` constant is already present in `Object`, it is not possible that the situation is

    module Admin
      User
    end

because otherwise Ruby would have resolved `User` and no autoloading would have been triggered in the first place. Thus, Rails assumes a qualified reference and considers the file `admin/user.rb` and directory `admin/user` to be the only valid options.

In practice, this works quite well as long as the nesting matches all parent namespaces respectively and the constants that make the rule apply are known at that time.

However, autoloading happens on demand. If by chance the top-level `User` was not yet loaded, then Rails assumes a relative reference by contract.

Naming conflicts of this kind are rare in practice, but if one occurs, `require_dependency` provides a solution by ensuring that the constant needed to trigger the heuristic is defined in the conflicting place.

### Automatic Modules¶ ↑

When a module acts as a namespace, Rails does not require the application to defines a file for it, a directory matching the namespace is enough.

Suppose an application has a back office whose controllers are stored in `app/controllers/admin`. If the `Admin` module is not yet loaded when `Admin::UsersController` is hit, Rails needs first to autoload the constant `Admin`.

If `autoload_paths` has a file called `admin.rb` Rails is going to load that one, but if there's no such file and a directory called `admin` is found, Rails creates an empty module and assigns it to the `Admin` constant on the fly.

### Generic Procedure¶ ↑

Relative references are reported to be missing in the cref where they were hit, and qualified references are reported to be missing in their parent (see Resolution Algorithm for Relative Constants at the beginning of this guide for the definition of cref, and Resolution Algorithm for Qualified Constants for the definition of parent).

The procedure to autoload constant `C` in an arbitrary situation is as follows:

    if the class or module in which C is missing is Object
      let ns = ''
    else
      let M = the class or module in which C is missing

      if M is anonymous
        let ns = ''
      else
        let ns = M.name
      end
    end

    loop do
      # Look for a regular file.
      for dir in autoload_paths
        if the file "#{dir}/#{ns.underscore}/c.rb" exists
          load/require "#{dir}/#{ns.underscore}/c.rb"

          if C is now defined
            return
          else
            raise LoadError
          end
        end
      end

      # Look for an automatic module.
      for dir in autoload_paths
        if the directory "#{dir}/#{ns.underscore}/c" exists
          if ns is an empty string
            let C = Module.new in Object and return
          else
            let C = Module.new in ns.constantize and return
          end
        end
      end

      if ns is empty
        # We reached the top-level without finding the constant.
        raise NameError
      else
        if C exists in any of the parent namespaces
          # Qualified constants heuristic.
          raise NameError
        else
          # Try again in the parent namespace.
          let ns = the parent namespace of ns and retry
        end
      end
    end

## require_dependency¶ ↑

Constant autoloading is triggered on demand and therefore code that uses a certain constant may have it already defined or may trigger an autoload. That depends on the execution path and it may vary between runs.

There are times, however, in which you want to make sure a certain constant is known when the execution reaches some code. `require_dependency` provides a way to load a file using the current loading mechanism, and keeping track of constants defined in that file as if they were autoloaded to have them reloaded as needed.

`require_dependency` is rarely needed, but see a couple of use-cases in Autoloading and STI and When Constants aren’t Triggered.

WARNING. Unlike autoloading, `require_dependency` does not expect the file to define any particular constant. Exploiting this behavior would be a bad practice though, file and constant paths should match.

## Constant Reloading¶ ↑

When `config.cache_classes` is false Rails is able to reload autoloaded constants.

For example, in you're in a console session and edit some file behind the scenes, the code can be reloaded with the `reload!` command:

    > reload!

When the application runs, code is reloaded when something relevant to this logic changes. In order to do that, Rails monitors a number of things:

- `config/routes.rb`.

- Locales.

- Ruby files under `autoload_paths`.

- `db/schema.rb` and `db/structure.sql`.

If anything in there changes, there is a middleware that detects it and reloads the code.

Autoloading keeps track of autoloaded constants. Reloading is implemented by removing them all from their respective classes and modules using `Module#remove_const`. That way, when the code goes on, those constants are going to be unknown again, and files reloaded on demand.

INFO. This is an all-or-nothing operation, Rails does not attempt to reload only what changed since dependencies between classes makes that really tricky. Instead, everything is wiped.

## Module#autoload isn't Involved¶ ↑

`Module#autoload` provides a lazy way to load constants that is fully integrated with the Ruby constant lookup algorithms, dynamic constant API, etc. It is quite transparent.

Rails internals make extensive use of it to defer as much work as possible from the boot process. But constant autoloading in Rails is not implemented with `Module#autoload`.

One possible implementation based on `Module#autoload` would be to walk the application tree and issue `autoload` calls that map existing file names to their conventional constant name.

There are a number of reasons that prevent Rails from using that implementation.

For example, `Module#autoload` is only capable of loading files using `require`, so reloading would not be possible. Not only that, it uses an internal `require` which is not `Kernel#require`.

Then, it provides no way to remove declarations in case a file is deleted. If a constant gets removed with `Module#remove_const` its `autoload` is not triggered again. Also, it doesn't support qualified names, so files with namespaces should be interpreted during the walk tree to install their own `autoload` calls, but those files could have constant references not yet configured.

An implementation based on `Module#autoload` would be awesome but, as you see, at least as of today it is not possible. Constant autoloading in Rails is implemented with `Module#const_missing`, and that's why it has its own contract, documented in this guide.

## Common Gotchas¶ ↑

### Nesting and Qualified Constants¶ ↑

Let's consider

    module Admin
      class UsersController < ApplicationController
        def index
          @users = User.all
        end
      end
    end

and

    class Admin::UsersController < ApplicationController
      def index
        @users = User.all
      end
    end

To resolve `User` Ruby checks `Admin` in the former case, but it does not in the latter because it does not belong to the nesting (see Nesting and Resolution Algorithms).

Unfortunately Rails autoloading does not know the nesting in the spot where the constant was missing and so it is not able to act as Ruby would. In particular, `Admin::User` will get autoloaded in either case.

Albeit qualified constants with `class` and `module` keywords may technically work with autoloading in some cases, it is preferable to use relative constants instead:

    module Admin
      class UsersController < ApplicationController
        def index
          @users = User.all
        end
      end
    end

### Autoloading and STI¶ ↑

Single Table Inheritance (STI) is a feature of Active Record that enables storing a hierarchy of models in one single table. The API of such models is aware of the hierarchy and encapsulates some common needs. For example, given these classes:

    # app/models/polygon.rb
    class Polygon < ActiveRecord::Base
    end

    # app/models/triangle.rb
    class Triangle < Polygon
    end

    # app/models/rectangle.rb
    class Rectangle < Polygon
    end

`Triangle.create` creates a row that represents a triangle, and `Rectangle.create` creates a row that represents a rectangle. If `id` is the ID of an existing record, `Polygon.find(id)` returns an object of the correct type.

Methods that operate on collections are also aware of the hierarchy. For example, `Polygon.all` returns all the records of the table, because all rectangles and triangles are polygons. Active Record takes care of returning instances of their corresponding class in the result set.

Types are autoloaded as needed. For example, if `Polygon.first` is a rectangle and `Rectangle` has not yet been loaded, Active Record autoloads it and the record is correctly instantiated.

All good, but if instead of performing queries based on the root class we need to work on some subclass, things get interesting.

While working with `Polygon` you do not need to be aware of all its descendants, because anything in the table is by definition a polygon, but when working with subclasses Active Record needs to be able to enumerate the types it is looking for. Let’s see an example.

`Rectangle.all` only loads rectangles by adding a type constraint to the query:

    SELECT "polygons".* FROM "polygons"
    WHERE "polygons"."type" IN ("Rectangle")

Let’s introduce now a subclass of `Rectangle`:

    # app/models/square.rb
    class Square < Rectangle
    end

`Rectangle.all` should now return rectangles and squares:

    SELECT "polygons".* FROM "polygons"
    WHERE "polygons"."type" IN ("Rectangle", "Square")

But there’s a caveat here: How does Active Record know that the class `Square` exists at all?

Even if the file `app/models/square.rb` exists and defines the `Square` class, if no code yet used that class, `Rectangle.all` issues the query

    SELECT "polygons".* FROM "polygons"
    WHERE "polygons"."type" IN ("Rectangle")

That is not a bug, the query includes all known descendants of `Rectangle`.

A way to ensure this works correctly regardless of the order of execution is to load the leaves of the tree by hand at the bottom of the file that defines the root class:

    # app/models/polygon.rb
    class Polygon < ActiveRecord::Base
    end
    require_dependency ‘square’

Only the leaves that are at least grandchildren need to be loaded this way. Direct subclasses do not need to be preloaded. If the hierarchy is deeper, intermediate classes will be autoloaded recursively from the bottom because their constant will appear in the class definitions as superclass.

### Autoloading and `require`¶ ↑

Files defining constants to be autoloaded should never be `require`d:

    require 'user' # DO NOT DO THIS

    class UsersController < ApplicationController
      ...
    end

There are two possible gotchas here in development mode:

1. If `User` is autoloaded before reaching the `require`, `app/models/user.rb` runs again because `load` does not update `$LOADED_FEATURES`.

2. If the `require` runs first Rails does not mark `User` as an autoloaded constant and changes to `app/models/user.rb` aren't reloaded.

Just follow the flow and use constant autoloading always, never mix autoloading and `require`. As a last resort, if some file absolutely needs to load a certain file use `require_dependency` to play nice with constant autoloading. This option is rarely needed in practice, though.

Of course, using `require` in autoloaded files to load ordinary 3rd party libraries is fine, and Rails is able to distinguish their constants, they are not marked as autoloaded.

### Autoloading and Initializers¶ ↑

Consider this assignment in `config/initializers/set_auth_service.rb`:

    AUTH_SERVICE = if Rails.env.production?
      RealAuthService
    else
      MockedAuthService
    end

The purpose of this setup would be that the application uses the class that corresponds to the environment via `AUTH_SERVICE`. In development mode `MockedAuthService` gets autoloaded when the initializer runs. Let’s suppose we do some requests, change its implementation, and hit the application again. To our surprise the changes are not reflected. Why?

As we saw earlier, Rails removes autoloaded constants, but `AUTH_SERVICE` stores the original class object. Stale, non-reachable using the original constant, but perfectly functional.

The following code summarizes the situation:

    class C
      def quack
        'quack!'
      end
    end

    X = C
    Object.instance_eval { remove_const(:C) }
    X.new.quack # => quack!
    X.name      # => C
    C           # => uninitialized constant C (NameError)

Because of that, it is not a good idea to autoload constants on application initialization.

In the case above we could implement a dynamic access point:

    # app/models/auth_service.rb
    class AuthService
      if Rails.env.production?
        def self.instance
          RealAuthService
        end
      else
        def self.instance
          MockedAuthService
        end
      end
    end

and have the application use `AuthService.instance` instead. `AuthService` would be loaded on demand and be autoload-friendly.

### `require_dependency` and Initializers¶ ↑

As we saw before, `require_dependency` loads files in an autoloading-friendly way. Normally, though, such a call does not make sense in an initializer.

One could think about doing some require_dependency calls in an initializer to make sure certain constants are loaded upfront, for example as an attempt to address the gotcha with STIs.

Problem is, in development mode autoloaded constants are wiped if there is any relevant change in the file system. If that happens then we are in the very same situation the initializer wanted to avoid!

Calls to `require_dependency` have to be strategically written in autoloaded spots.

### When Constants aren't Missed¶ ↑

#### Relative References¶ ↑

Let's consider a flight simulator. The application has a default flight model

    # app/models/flight_model.rb
    class FlightModel
    end

that can be overridden by each airplane, for instance

    # app/models/bell_x1/flight_model.rb
    module BellX1
      class FlightModel < FlightModel
      end
    end

    # app/models/bell_x1/aircraft.rb
    module BellX1
      class Aircraft
        def initialize
          @flight_model = FlightModel.new
        end
      end
    end

The initializer wants to create a `BellX1::FlightModel` and nesting has `BellX1`, that looks good. But if the default flight model is loaded and the one for the Bell-X1 is not, the interpreter is able to resolve the top-level `FlightModel` and autoloading is thus not triggered for `BellX1::FlightModel`.

That code depends on the execution path.

These kind of ambiguities can often be resolved using qualified constants:

    module BellX1
      class Plane
        def flight_model
          @flight_model ||= BellX1::FlightModel.new
        end
      end
    end

Also, `require_dependency` is a solution:

    require_dependency 'bell_x1/flight_model'

    module BellX1
      class Plane
        def flight_model
          @flight_model ||= FlightModel.new
        end
      end
    end

#### Qualified References¶ ↑

Given

    # app/models/hotel.rb
    class Hotel
    end

    # app/models/image.rb
    class Image
    end

    # app/models/hotel/image.rb
    class Hotel
      class Image < Image
      end
    end

the expression `Hotel::Image` is ambiguous because it depends on the execution path.

As we saw before, Ruby looks up the constant in `Hotel` and its ancestors. If `app/models/image.rb` has been loaded but `app/models/hotel/image.rb` hasn't, Ruby does not find `Image` in `Hotel`, but it does in `Object`:

    $ bin/rails r 'Image; p Hotel::Image' 2>/dev/null
    Image # NOT Hotel::Image!

The code evaluating `Hotel::Image` needs to make sure `app/models/hotel/image.rb` has been loaded, possibly with `require_dependency`.

In these cases the interpreter issues a warning though:

    warning: toplevel constant Image referenced by Hotel::Image

This surprising constant resolution can be observed with any qualifying class:

    2.1.5 :001 > String::Array
    (irb):1: warning: toplevel constant Array referenced by String::Array
     => Array

WARNING. To find this gotcha the qualifying namespace has to be a class, `Object` is not an ancestor of modules.

### Autoloading within Singleton Classes¶ ↑

Let's suppose we have these class definitions:

    # app/models/hotel/services.rb
    module Hotel
      class Services
      end
    end

    # app/models/hotel/geo_location.rb
    module Hotel
      class GeoLocation
        class << self
          Services
        end
      end
    end

If `Hotel::Services` is known by the time `app/models/hotel/geo_location.rb` is being loaded, `Services` is resolved by Ruby because `Hotel` belongs to the nesting when the singleton class of `Hotel::GeoLocation` is opened.

But if `Hotel::Services` is not known, Rails is not able to autoload it, the application raises `NameError`.

The reason is that autoloading is triggered for the singleton class, which is anonymous, and as we saw before, Rails only checks the top-level namespace in that edge case.

An easy solution to this caveat is to qualify the constant:

    module Hotel
      class GeoLocation
        class << self
          Hotel::Services
        end
      end
    end

### Autoloading in `BasicObject`¶ ↑

Direct descendants of `BasicObject` do not have `Object` among their ancestors and cannot resolve top-level constants:

    class C < BasicObject
      String # NameError: uninitialized constant C::String
    end

When autoloading is involved that plot has a twist. Let's consider:

    class C < BasicObject
      def user
        User # WRONG
      end
    end

Since Rails checks the top-level namespace `User` gets autoloaded just fine the first time the `user` method is invoked. You only get the exception if the `User` constant is known at that point, in particular in a second call to `user`:

    c = C.new
    c.user # surprisingly fine, User
    c.user # NameError: uninitialized constant C::User

because it detects that a parent namespace already has the constant (see Qualified References).

As with pure Ruby, within the body of a direct descendant of `BasicObject` use always absolute constant paths:

    class C < BasicObject
      ::String # RIGHT

      def user
        ::User # RIGHT
      end
    end