AcitveRecord不會爲具有嵌套現有記錄的新關聯記錄accept_nested_attributes_

Question

ActiveRecord似乎並不理解，給定一組具有嵌套屬性的現有記錄的參數，它可以創建一個新的嵌套記錄，該記錄本身具有嵌套的現有記錄。 （關係樹：Existing -> New -> Existing）

這是一個錯誤，還是我錯過了什麼？

讓我給你看一個簡單的例子。

這裏是我的模型：

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 

我做了每個表中的一個記錄，因此它們有關的，所以每個表都有一個記錄在其與id=1 - 這是已知的。現在，如果我有一個現有的用戶，新郵，和現有的組，並嘗試更新使用accepts_nested_attributes_for該記錄，它不喜歡它：

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 > 

它認爲它不能找到一組（有一個已知的ID），與一個新的Post相關。 它可以在我從group_attributes中刪除ID時生效（但會創建一個新的組記錄）。 當我給posts_attributes一個ID時，它會起作用，並從group_attributes中刪除該ID（並再次創建一個新組）。 它也適用於全部都有ID的情況。

的關係工作：

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創建過程中使用posts_attributes和group_attributes時，如果所有的記錄都新也完全適用。

不應該在第一個例子中工作嗎？ ActiveRecord應該足夠聰明以解決這個問題......！

Answer 1

以下是我認爲正在發生的事情：您正在爲組傳遞一個ID，指示ActiveRecord該組存在。 ActiveRecord正試圖找到該組以使用group_attributes中的其他數據對其進行更新。既然你這樣做是內的post_attributes，ActiveRecord的是試圖通過後和組之間的關聯找到該組。也就是說，ActiveRecord首先查找關聯組 - 其中id = post.group_id - 然後從該結果中查找ID = 1的組織。對於父母關係來說，這看起來有點奇怪和笨拙，就像你的情況一樣。我相信你可以看到，在朝着另一個方向前進時這是有用的行爲，其中嵌套屬性表示一個或多個潛在的許多孩子。

但是，根據post_attributes中的數據創建的發佈對象尚未與組關聯 - post.group_id爲零。所以，當ActiveRecord首先進行搜索以獲取關聯組時，它就會變空。相應地，它在（空）結果中找不到ID = 1的組。從技術上講，該記錄在那裏，但在之後，它不存在。

您可以通過在post_attributes中包含group_id => 1來證明這一點。我相信如果你這樣做，ActiveRecord將通過關聯找到組，然後從結果中成功選擇ID = 1的組，然後使用group_attributes中的其他數據更新該組。

還要注意的是，只有原因包括嵌套屬性的組內後，就像你正在做的是，如果你是允許用戶在創建一個新的職位的同時更新組名。如果您所要做的只是將新帖子鏈接到現有的羣組，那麼您只需要在post_attributes中包含group_id，並且可以擺脫group_attributes。