Module: ActiveRecord::NestedAttributes::ClassMethods

Defined in:: activerecord/lib/active_record/nested_attributes.rb

Overview

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

The belongs_to association validates the presence of the parent model by default. You can disable this behavior by specifying optional: true. This can be used, for example, when conditionally validating the presence of the parent model:

class Veterinarian < ActiveRecord::Base
  has_many :patients, inverse_of: :veterinarian
  accepts_nested_attributes_for :patients
end

class Patient < ActiveRecord::Base
  belongs_to :veterinarian, inverse_of: :patients, optional: true
  validates :veterinarian, presence: true, unless: -> { awaiting_intake }
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

Creating forms with nested attributes

Use ActionView::Helpers::FormHelper#fields_for to create form elements for updating or destroying nested attributes.

Testing

If you are using ActionView::Helpers::FormHelper#fields_for, your integration tests should replicate the HTML structure it provides. For example;

post members_path, params: {
  member: {
    name: 'joe',
    posts_attributes: {
      '0' => { title: 'Foo' },
      '1' => { title: 'Bar' }
    }
  }
}

Constant Summary collapse

REJECT_ALL_BLANK_PROC =

proc { |attributes| attributes.all? { |key, value| key == "_destroy" || value.blank? } }

Instance Method Summary collapse

#accepts_nested_attributes_for(*attr_names) ⇒ Object

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

Instance Method Details

#accepts_nested_attributes_for(*attr_names) ⇒ `Object`

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 (e.g. 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 'activerecord/lib/active_record/nested_attributes.rb', line 353

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