AcitveRecord不会为具有嵌*有记录的新关联记录accept_nested_attributes_

Question

ActiveRecord似乎并不理解，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。

这里是我的模型：

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 

我做了每个表中的一个记录，因此它们有关的，所以每个表都有一个记录在其与id=1 - 这是已知的。现在，如果我有一个现有的用户，新邮，和现有的组，并尝试更新使用accepts_nested_attributes_for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 > 

它认为它不能找到一组（有一个已知的ID），与一个新的Post相关。 它可以在我从group_attributes中删除ID时生效（但会创建一个新的组记录）。 当我给posts_attributes一个ID时，它会起作用，并从group_attributes中删除该ID（并再次创建一个新组）。 它也适用于全部都有ID的情况。

的关系工作：

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 
    } 
] 

User创建过程中使用posts_attributes和group_attributes时，如果所有的记录都新也完全适用。

不应该在第一个例子中工作吗？ ActiveRecord应该足够聪明以解决这个问题......！

Answer 1

以下是我认为正在发生的事情：您正在为组传递一个ID，指示ActiveRecord该组存在。 ActiveRecord正试图找到该组以使用group_attributes中的其他数据对其进行更新。既然你这样做是内的post_attributes，ActiveRecord的是试图通过后和组之间的关联找到该组。也就是说，ActiveRecord首先查找关联组 - 其中id = post.group_id - 然后从该结果中查找ID = 1的组织。对于父母关系来说，这看起来有点奇怪和笨拙，就像你的情况一样。我相信你可以看到，在朝着另一个方向前进时这是有用的行为，其中嵌套属性表示一个或多个潜在的许多孩子。

但是，根据post_attributes中的数据创建的发布对象尚未与组关联 - post.group_id为零。所以，当ActiveRecord首先进行搜索以获取关联组时，它就会变空。相应地，它在（空）结果中找不到ID = 1的组。从技术上讲，该记录在那里，但在之后，它不存在。

您可以通过在post_attributes中包含group_id => 1来证明这一点。我相信如果你这样做，ActiveRecord将通过关联找到组，然后从结果中成功选择ID = 1的组，然后使用group_attributes中的其他数据更新该组。

还要注意的是，只有原因包括嵌套属性的组内后，就像你正在做的是，如果你是允许用户在创建一个新的职位的同时更新组名。如果您所要做的只是将新帖子链接到现有的群组，那么您只需要在post_attributes中包含group_id，并且可以摆脱group_attributes。