複数の子レコードを作成、更新する

1対1の時

MemberモデルはAbatarモデルに１対１の関連を持っている

app/assets/models/member.rb

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

app/assets/controllers/members_controller.rb

def create
  params = { member: { name: 'Jack', avatar_attributes: { icon: 'smiling' } } }
  member = Member.create(params[:member])
  member.avatar.id # => 2
  member.avatar.icon # => 'smiling'
end

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

def destroy
  member.avatar_attributes = { id: '2', _destroy: '1' }
  member.avatar.marked_for_destruction? # => true
  member.save
  member.reload.avatar # => nil
end

1対多の時

MemberモデルはPostモデルに１対多の関連を持っている

app/assets/models/member.rb

class Member < ActiveRecord::Base
  has_many :posts
  accepts_nested_attributes_for :posts, allow_destroy: true
end

app/assets/controllers/members_controller.rb

def create
  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'
end

def update
  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'
end

def destroy
  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
end

reject_if

パラメータが指定した条件に合致した場合、子要素を作成しない.
procとmethodのシンボルで指定可能

proc

app/assets/models/member.rb

class Member < ActiveRecord::Base
  has_many :posts
  accepts_nested_attributes_for :posts, reject_if: proc { |attributes| attributes['title'].blank? }
end

method

app/assets/models/member.rb

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

参考

複数の子レコードを作成・更新する. accepts_nested_attributes_for
http://qiita.com/hmuronaka/items/818c421dc632e3efb7a6#3-1