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'

If you want to update the current avatar without providing the id, you must add :update_only option.

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

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

activerecord/lib/active_record/nested_attributes.rb