ruby-on-rails - AcitveRecord 不会接受_nested_attributes_for 具有嵌套现有记录的新关联记录

Question

ActiveRecord 似乎不明白，给定一组具有嵌套属性的现有记录的参数，它可以创建一个新的嵌套记录，该记录本身具有嵌套的现有记录。 (关系树:Existing -> New -> Existing)

这是一个错误，还是我错过了什么？

让我给你看一个简单的例子。

这是我的模型:

class User < ActiveRecord::Base
  has_many :posts
  attr_accessible :name, :posts_attributes
  accepts_nested_attributes_for :posts
end

class Post < ActiveRecord::Base
  belongs_to :group
  belongs_to :user
  attr_accessible :content, :title, :group_attributes
  accepts_nested_attributes_for :group
end

class Group < ActiveRecord::Base
  has_many :posts
  attr_accessible :name
end

我在每个表中创建了一条记录，并相应地将它们关联起来，因此每个表中都有一条记录，带有 id=1 ——这是众所周知的。现在，如果我有一个现有用户、一个新帖子和一个现有组，并尝试使用 accepts_nested_attributes_for 更新该记录。 ，它不喜欢它:

1.9.3-p125 :044 > params
{
                  :id => 1,
                :name => "Billy",
    :posts_attributes => [
        [0] {
                          :title => "Title",
                        :content => "Some magnificent content for you!",
            :group_attributes => {
                  :id => 1,
                :name => "Group 1"
            }
        }
    ]
}
1.9.3-p125 :045 > u
#<User:0x00000002f7f380> {
            :id => 1,
          :name => "Billy",
    :created_at => Fri, 03 Aug 2012 20:21:37 UTC +00:00,
    :updated_at => Fri, 03 Aug 2012 20:21:37 UTC +00:00
}
1.9.3-p125 :046 > u.update_attributes params
   (0.1ms)  begin transaction
   (0.1ms)  rollback transaction
ActiveRecord::RecordNotFound: Couldn't find Group with ID=1 for Post with ID=
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:462:in `raise_nested_attributes_record_not_found'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:332:in `assign_nested_attributes_for_one_to_one_association'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:288:in `group_attributes='
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:94:in `block in assign_attributes'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:93:in `each'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:93:in `assign_attributes'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/base.rb:498:in `initialize'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/reflection.rb:183:in `new'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/reflection.rb:183:in `build_association'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/associations/association.rb:233:in `build_record'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/associations/collection_association.rb:112:in `build'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:405:in `block in assign_nested_attributes_for_collection_association'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:400:in `each'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:400:in `assign_nested_attributes_for_collection_association'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/nested_attributes.rb:288:in `posts_attributes='
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:85:in `block in assign_attributes'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:78:in `each'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/attribute_assignment.rb:78:in `assign_attributes'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/persistence.rb:216:in `block in update_attributes'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/transactions.rb:295:in `block in with_transaction_returning_status'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/connection_adapters/abstract/database_statements.rb:192:in `transaction'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/transactions.rb:208:in `transaction'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/activerecord-3.2.7/lib/active_record/persistence.rb:215:in `update_attributes'
  from (irb):15
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.7/lib/rails/commands/console.rb:47:in `start'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.7/lib/rails/commands/console.rb:8:in `start'
  from /home/trevor/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.7/lib/rails/commands.rb:41:in `<top (required)>'
  from script/rails:6:in `require'
  from script/rails:6:in `<main>'1.9.3-p125 :047 > 

它认为它找不到与新帖子相关的组(具有已知 ID)。
当我从 group_attributes 中删除 ID 时它会起作用(但它创建了一个新的组记录)。
当我给 posts_attributes 时它有效一个 ID，并从 group_attributes 中删除该 ID (并再次创建一个新组)。
当他们都有 ID 时它也有效。

关系正在运行:

1.9.3-p125 :059 > p = Post.new( { group_attributes: { name: 'Testing' } } )
#<Post:0x00000004212380> {
            :id => nil,
         :title => nil,
       :content => nil,
      :group_id => nil,
       :user_id => nil,
    :created_at => nil,
    :updated_at => nil
}
1.9.3-p125 :060 > p.group
[
    [0] #<Group:0x00000004211868> {
                :id => nil,
              :name => "Testing",
        :created_at => nil,
        :updated_at => nil
    }
]

使用 posts_attributes 时它也完全有效和 group_attributes期间User创建，如果所有记录都是新的。

它不应该在第一个例子中仍然有效吗？ ActiveRecord 应该足够聪明来解决这个问题......!

Answer 1

这是我认为正在发生的事情:您正在传递一个组的 ID，向 ActiveRecord 表明该组存在。 ActiveRecord 正在尝试查找该组以使用您在 中拥有的其他数据来更新它。 group_attributes .由于您是在 内部执行此操作post_attributes , ActiveRecord 正在尝试查找该组 通过协会在帖子和群组之间。也就是说，ActiveRecord 首先查找关联的组 - 其中 id = post.group_id - 然后从该结果中查找 ID = 1 的组。对于父关系，这可能看起来有点奇怪和笨拙，就像您的情况一样，但是我敢肯定，当您从另一个方向前进时，您会发现这是一种有用的行为，其中嵌套的属性代表一个或多个潜在的多个子项。

但是，根据 post_attributes 中的数据创建的 post 对象尚未与组关联 - post.group_id 为零。因此，当 ActiveRecord 执行第一次搜索以获取关联的组时，它会显示为空。相应地，它在(空)结果中找不到 ID = 1 的组。从技术上讲，记录是存在的，但就与帖子的关联而言，它并不存在。

您可以通过在 post_attributes 中包含 group_id => 1 来证明这一点。我相信如果这样做，ActiveRecord 会通过关联找到组，然后从结果中子选择 ID = 1 的组，成功，然后使用 group_attributes 中的附加数据更新该组。

另请注意，唯一的原因如果您允许用户在创建新帖子的同时更新组名称，则在帖子内包含组的嵌套属性。如果您想要做的只是将新帖子链接到现有组，那么您只需要在 post_attributes 中包含 group_id 即可摆脱 group_attributes。