Nested Attributes

Nested attributes allow you to save attributes on associated records through the parent. By default nested attribute updating is turned off, 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.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_attributes 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.avatar #=> nil

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

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 an associated post model through the attribute hash.

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(attributed)
       attributed['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'

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.posts.length # => 1

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 parents save method. See ActiveRecord::AutosaveAssociation.

  # 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 activerecord/lib/active_record/nested_attributes.rb, line 226
226:       def accepts_nested_attributes_for(*attr_names)
227:         options = { :allow_destroy => false, :update_only => false }
228:         options.update(attr_names.extract_options!)
229:         options.assert_valid_keys(:allow_destroy, :reject_if, :limit, :update_only)
230:         options[:reject_if] = REJECT_ALL_BLANK_PROC if options[:reject_if] == :all_blank
231: 
232:         attr_names.each do |association_name|
233:           if reflection = reflect_on_association(association_name)
234:             reflection.options[:autosave] = true
235:             add_autosave_association_callbacks(reflection)
236:             nested_attributes_options[association_name.to_sym] = options
237:             type = (reflection.collection? ? :collection : :one_to_one)
238: 
239:             # def pirate_attributes=(attributes)
240:             #   assign_nested_attributes_for_one_to_one_association(:pirate, attributes)
241:             # end
242:             class_eval "def \#{association_name}_attributes=(attributes)\nassign_nested_attributes_for_\#{type}_association(:\#{association_name}, attributes)\nend\n", __FILE__, __LINE__ + 1
243:           else
244:             raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
245:           end
246:         end
247:       end