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.
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.
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.
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.
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
# File activerecord/lib/active_record/nested_attributes.rb, line 333 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
Defines an attributes writer for the specified association(s).
Supported options:
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.
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
.
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.
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
© 2004–2019 David Heinemeier Hansson
Licensed under the MIT License.