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, 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

Constant Summary collapse

REJECT_ALL_BLANK_PROC = 
proc { |attributes| attributes.all? { |_, value| value.blank? } }

Instance Method Summary collapse

Instance Method Details

#accepts_nested_attributes_for(*attr_names) ⇒ Object

Defines an attributes writer for the specified association(s). If you are using attr_protected or attr_accessible, then you will need to add the attribute writer to the allowed list.

Supported options:

:allow_destroy

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.

: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.

:limit

Allows you to specify the maximum number of the associated records that can be processed with the nested attributes. If the size of the nested attributes array exceeds the specified limit, NestedAttributes::TooManyRecords exception is raised. If omitted, any number associations can be processed. Note that the :limit option is only applicable to one-to-many associations.

:update_only

Allows you to specify that an existing record may only be updated. A new record may only be created when there is no existing record. This option only works for one-to-one associations and is ignored for collection associations. This option is off by default.

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



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# File 'activerecord/lib/active_record/nested_attributes.rb', line 262

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.options[:autosave] = true
      add_autosave_association_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)

      # def pirate_attributes=(attributes)
      #   assign_nested_attributes_for_one_to_one_association(:pirate, attributes, mass_assignment_options)
      # end
      class_eval <<-eoruby, __FILE__, __LINE__ + 1
        if method_defined?(:#{association_name}_attributes=)
          remove_method(:#{association_name}_attributes=)
        end
        def #{association_name}_attributes=(attributes)
          assign_nested_attributes_for_#{type}_association(:#{association_name}, attributes, mass_assignment_options)
        end
      eoruby
    else
      raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
    end
  end
end