# module ActiveRecord::NestedAttributes::ClassMethods

# Active Record Nested Attributes¶ ↑

Nested attributes allow you to save attributes on associated records through the parent. By default nested attribute updating is turned off and you can enable it using the #accepts_nested_attributes_for class method. When you enable nested attributes an attribute writer is defined on the model.

The attribute writer is named after the association, which means that in the following example, two new methods are added to your model:

`author_attributes=(attributes)` and `pages_attributes=(attributes)`.

    class Book < ActiveRecord::Base
      has_one :author
      has_many :pages

      accepts_nested_attributes_for :author, :pages
    end

Note that the `:autosave` option is automatically enabled on every association that #accepts_nested_attributes_for is used for.

### One-to-one¶ ↑

Consider a Member model that has one Avatar:

    class Member < ActiveRecord::Base
      has_one :avatar
      accepts_nested_attributes_for :avatar
    end

Enabling nested attributes on a one-to-one association allows you to create the member and avatar in one go:

    params = { member: { name: 'Jack', avatar_attributes: { icon: 'smiling' } } }
    member = Member.create(params[:member])
    member.avatar.id # => 2
    member.avatar.icon # => 'smiling'

It also allows you to update the avatar through the member:

    params = { member: { avatar_attributes: { id: '2', icon: 'sad' } } }
    member.update params[:member]
    member.avatar.icon # => 'sad'

By default you will only be able to set and update attributes on the associated model. If you want to destroy the associated model through the attributes hash, you have to enable it first using the `:allow_destroy` option.

    class Member < ActiveRecord::Base
      has_one :avatar
      accepts_nested_attributes_for :avatar, allow_destroy: true
    end

Now, when you add the `_destroy` key to the attributes hash, with a value that evaluates to `true`, you will destroy the associated model:

    member.avatar_attributes = { id: '2', _destroy: '1' }
    member.avatar.marked_for_destruction? # => true
    member.save
    member.reload.avatar # => nil

Note that the model will not be destroyed until the parent is saved.

Also note that the model will not be destroyed unless you also specify its id in the updated hash.

### One-to-many¶ ↑

Consider a member that has a number of posts:

    class Member < ActiveRecord::Base
      has_many :posts
      accepts_nested_attributes_for :posts
    end

You can now set or update attributes on the associated posts through an attribute hash for a member: include the key `:posts_attributes` with an array of hashes of post attributes as a value.

For each hash that does not have an `id` key a new record will be instantiated, unless the hash also contains a `_destroy` key that evaluates to `true`.

    params = { member: {
      name: 'joe', posts_attributes: [
        { title: 'Kari, the awesome Ruby documentation browser!' },
        { title: 'The egalitarian assumption of the modern citizen' },
        { title: '', _destroy: '1' } # this will be ignored
      ]
    }}

    member = Member.create(params[:member])
    member.posts.length # => 2
    member.posts.first.title # => 'Kari, the awesome Ruby documentation browser!'
    member.posts.second.title # => 'The egalitarian assumption of the modern citizen'

You may also set a `:reject_if` proc to silently ignore any new record hashes if they fail to pass your criteria. For example, the previous example could be rewritten as:

    class Member < ActiveRecord::Base
      has_many :posts
      accepts_nested_attributes_for :posts, reject_if: proc { |attributes| attributes['title'].blank? }
    end

    params = { member: {
      name: 'joe', posts_attributes: [
        { title: 'Kari, the awesome Ruby documentation browser!' },
        { title: 'The egalitarian assumption of the modern citizen' },
        { title: '' } # this will be ignored because of the :reject_if proc
      ]
    }}

    member = Member.create(params[:member])
    member.posts.length # => 2
    member.posts.first.title # => 'Kari, the awesome Ruby documentation browser!'
    member.posts.second.title # => 'The egalitarian assumption of the modern citizen'

Alternatively, `:reject_if` also accepts a symbol for using methods:

    class Member < ActiveRecord::Base
      has_many :posts
      accepts_nested_attributes_for :posts, reject_if: :new_record?
    end

    class Member < ActiveRecord::Base
      has_many :posts
      accepts_nested_attributes_for :posts, reject_if: :reject_posts

      def reject_posts(attributes)
        attributes['title'].blank?
      end
    end

If the hash contains an `id` key that matches an already associated record, the matching record will be modified:

    member.attributes = {
      name: 'Joe',
      posts_attributes: [
        { id: 1, title: '[UPDATED] An, as of yet, undisclosed awesome Ruby documentation browser!' },
        { id: 2, title: '[UPDATED] other post' }
      ]
    }

    member.posts.first.title # => '[UPDATED] An, as of yet, undisclosed awesome Ruby documentation browser!'
    member.posts.second.title # => '[UPDATED] other post'

However, the above applies if the parent model is being updated as well. For example, If you wanted to create a `member` named joe and wanted to update the `posts` at the same time, that would give an ActiveRecord::RecordNotFound error.

By default the associated records are protected from being destroyed. If you want to destroy any of the associated records through the attributes hash, you have to enable it first using the `:allow_destroy` option. This will allow you to also use the `_destroy` key to destroy existing records:

    class Member < ActiveRecord::Base
      has_many :posts
      accepts_nested_attributes_for :posts, allow_destroy: true
    end

    params = { member: {
      posts_attributes: [{ id: '2', _destroy: '1' }]
    }}

    member.attributes = params[:member]
    member.posts.detect { |p| p.id == 2 }.marked_for_destruction? # => true
    member.posts.length # => 2
    member.save
    member.reload.posts.length # => 1

Nested attributes for an associated collection can also be passed in the form of a hash of hashes instead of an array of hashes:

    Member.create(
      name: 'joe',
      posts_attributes: {
        first:  { title: 'Foo' },
        second: { title: 'Bar' }
      }
    )

has the same effect as

    Member.create(
      name: 'joe',
      posts_attributes: [
        { title: 'Foo' },
        { title: 'Bar' }
      ]
    )

The keys of the hash which is the value for `:posts_attributes` are ignored in this case. However, it is not allowed to use `'id'` or `:id` for one of such keys, otherwise the hash will be wrapped in an array and interpreted as an attribute hash for a single post.

Passing attributes for an associated collection in the form of a hash of hashes can be used with hashes generated from HTTP/HTML parameters, where there may be no natural way to submit an array of hashes.

### Saving¶ ↑

All changes to models, including the destruction of those marked for destruction, are saved and destroyed automatically and atomically when the parent model is saved. This happens inside the transaction initiated by the parent's save method. See ActiveRecord::AutosaveAssociation.

### Validating the presence of a parent model¶ ↑

If you want to validate that a child record is associated with a parent record, you can use the `validates_presence_of` method and the `:inverse_of` key as this example illustrates:

    class Member < ActiveRecord::Base
      has_many :posts, inverse_of: :member
      accepts_nested_attributes_for :posts
    end

    class Post < ActiveRecord::Base
      belongs_to :member, inverse_of: :posts
      validates_presence_of :member
    end

Note that if you do not specify the `:inverse_of` option, then Active Record will try to automatically guess the inverse association based on heuristics.

For one-to-one nested associations, if you build the new (in-memory) child object yourself before assignment, then this module will not overwrite it, e.g.:

    class Member < ActiveRecord::Base
      has_one :avatar
      accepts_nested_attributes_for :avatar

      def avatar
        super || build_avatar(width: 200)
      end
    end

    member = Member.new
    member.avatar_attributes = {icon: 'sad'}
    member.avatar.width # => 200

### Constants

REJECT_ALL_BLANK_PROC

### Public Instance Methods

accepts_nested_attributes_for(*attr_names) click to toggle source

Defines an attributes writer for the specified association(s).

Supported options:

:allow_destroy

  If true, destroys any members from the attributes hash with a `_destroy` key and a value that evaluates to `true` (eg. 1, '1', true, or 'true'). This option is off by default.

:reject_if

  Allows you to specify a Proc or a Symbol pointing to a method that checks whether a record should be built for a certain attribute hash. The hash is passed to the supplied Proc or the method and it should return either `true` or `false`. When no `:reject_if` is specified, a record will be built for all attribute hashes that do not have a `_destroy` value that evaluates to true. Passing `:all_blank` instead of a Proc will create a proc that will reject a record where all the attributes are blank excluding any value for `_destroy`.

:limit

  Allows you to specify the maximum number of associated records that can be processed with the nested attributes. Limit also can be specified as a Proc or a Symbol pointing to a method that should return a number. If the size of the nested attributes array exceeds the specified limit, NestedAttributes::TooManyRecords exception is raised. If omitted, any number of associations can be processed. Note that the `:limit` option is only applicable to one-to-many associations.

:update_only

  For a one-to-one association, this option allows you to specify how nested attributes are going to be used when an associated record already exists. In general, an existing record may either be updated with the new set of attribute values or be replaced by a wholly new record containing those values. By default the `:update_only` option is `false` and the nested attributes are used to update the existing record only if they include the record's `:id` value. Otherwise a new record will be instantiated and used to replace the existing one. However if the `:update_only` option is `true`, the nested attributes are used to update the record's attributes always, regardless of whether the `:id` is present. The option is ignored for collection associations.

Examples:

    # creates avatar_attributes=
    accepts_nested_attributes_for :avatar, reject_if: proc { |attributes| attributes['name'].blank? }
    # creates avatar_attributes=
    accepts_nested_attributes_for :avatar, reject_if: :all_blank
    # creates avatar_attributes= and posts_attributes=
    accepts_nested_attributes_for :avatar, :posts, allow_destroy: true

    # File lib/active_record/nested_attributes.rb, line 319
    def accepts_nested_attributes_for(*attr_names)
      options = { allow_destroy: false, update_only: false }
      options.update(attr_names.extract_options!)
      options.assert_valid_keys(:allow_destroy, :reject_if, :limit, :update_only)
      options[:reject_if] = REJECT_ALL_BLANK_PROC if options[:reject_if] == :all_blank

      attr_names.each do |association_name|
        if reflection = _reflect_on_association(association_name)
          reflection.autosave = true
          define_autosave_validation_callbacks(reflection)

          nested_attributes_options = self.nested_attributes_options.dup
          nested_attributes_options[association_name.to_sym] = options
          self.nested_attributes_options = nested_attributes_options

          type = (reflection.collection? ? :collection : :one_to_one)
          generate_association_writer(association_name, type)
        else
          raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
        end
      end
    end

### Private Instance Methods

generate_association_writer(association_name, type) click to toggle source

Generates a writer method for this association. Serves as a point for accessing the objects in the association. For example, this method could generate the following:

    def pirate_attributes=(attributes)
      assign_nested_attributes_for_one_to_one_association(:pirate, attributes)
    end

This redirects the attempts to write objects in an association through the helper methods defined below. Makes it seem like the nested associations are just regular associations.

    # File lib/active_record/nested_attributes.rb, line 355
            def generate_association_writer(association_name, type)
              generated_association_methods.module_eval "            if method_defined?(:#{association_name}_attributes=)
                  remove_method(:#{association_name}_attributes=)
                end
                def #{association_name}_attributes=(attributes)
                  assign_nested_attributes_for_#{type}_association(:#{association_name}, attributes)
                end
    ", __FILE__, __LINE__ + 1
            end