accepts_nested_attributes_for

通过嵌套属性（nested attribute），你可以通过parent来保存与其相关联的属性。默认情况下，嵌套属性是关闭的，你可以开启accepts_nested_attributes_for这个类方法，就在该model上生成一个属性writer。
属性writer是以该关联命名。例如，为你的model增加两个新方法：

author_attributes=(attributes) 和 pages_attributes=(attributes).

 class Book < ActiveRecord::Base
    has_one :author
    has_many :pages

    accepts_nested_attributes_for :author, :pages
end

 
使用了accepts_nested_attributes_for的每一个关联都自动开启:autosave

一对一关联

一个Member有一个Avatar

class Member < ActiveRecord::Base
  has_one :avatar
  accepts_nested_attributes_for :avatar
end

 
开启一对一关联的嵌套属性可以通过这样方法一次性创建Member：

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

也可以这样update avatar

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

默认情况下你只能设置或更新关联的model。如果你想通过属性hash来删除关联model，你需要使用:allow_destroy 选项

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

此时，如果向属性hash里增加一个_destroy的key，并且value是true，则该关联model将被删除。

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

 
注意这里，只有当parent被保存后，关联的model才真正被删除。

一对多关联

一个member有一些post

class Member < ActiveRecord::Base
  has_many :posts
  accepts_nested_attributes_for :posts
end

你可通过属性hash来增加或更新关联post model
每一个不含有id键的新记录会被实例化，除非该hash也包含了一个 :_destroy => 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' } # 该记录会被忽略
    ]
  }
}
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'

 
通过 :reject_if proc 设置忽略的不满足条件的记录。例如：

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 => '' } #  这个记录会被忽略
  ]
}}

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'

:reject_if 也可一接受一个symbol来代表一个可用方法：
如果hash中含有一个id和已有的关联记录相匹配，则被匹配到的记录会被修改：

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'

默认关联记录是被保护的（不被删除）。如果想通过属性hash来删除任何关联记录，你需要打开:allow_destroy选项, 这样使用_destroy键来删除记录就行:

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  标记将被删除
member.posts.length # => 2  未保存，所以还没删除
member.save
member.reload.posts.length # => 1

保存

所有对model修改的行为，包括标记将要销毁，会随着parent被保存原子性的被自动保存或者删除。这些开始与parent的save方法，发生在其内部的事务里。详见 Active Record Autosave Association

使用attr_accessible

使用attr_accessible如果不小心，可能会干扰到嵌套属性的使用。例如，上述member model如下使用attr_accessible：

attr_accessible :name

 
你需要这样的修改：

attr_accessible :name, :posts_attributes

验证parent model 的存在

如果你想验证一个child记录是否和一个parent记录关联，你可以使用validates_presence_of和inverse_of：

<span style="font-size: xx-small;">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</span>