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, 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_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.reload.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.reload.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.

Using with attr_accessible

The use of attr_accessible can interfere with nested attributes if you’re not careful. For example, if the Member model above was using attr_accessible like this:

attr_accessible :name

You would need to modify it to look like this:

attr_accessible :name, :posts_attributes

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 validates_presence_of and inverse_of 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