ActiveRecord似乎不明白，为具有嵌套属性的现有记录指定一组参数，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bug，还是我遗漏了什么？
让我给你看一个简单的例子。
以下是我的型号：
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个记录，并相应地将它们关联起来，所以每个表中都有一个带有id=1的记录——这是已知的。现在，如果我有一个现有的User、一个新Post和一个现有Group，并尝试使用accepts_nested_attributes_for更新该记录，它不喜欢：
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 >
它认为找不到与新帖子相关的群组（具有已知ID）。当我从group_attributes中删除ID时，它就起作用了（但它会创建一个新的组记录）。当我给posts_attributes一个ID，并从group_attributes中删除该ID（然后再次创建一个新组）时，它就起作用了。当他们都有ID时，它也会起作用。
这种关系正在发挥作用：
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
}
]
如果所有记录都是新的，那么在创建User期间使用posts_attributes和group_attributes时，它也完全有效。
它在第一个例子中不应该仍然有效吗？ActiveRecord应该足够聪明，能够解决这个问题。。。！
以下是我认为正在发生的事情：您正在传递一个组的ID，向ActiveRecord指示该组存在。ActiveRecord正在尝试查找该组，以便使用group_attributes中的其他数据对其进行更新。由于您在post_attributes内部执行，ActiveRecord正试图通过post和组之间的关联来查找该组。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。对于父母关系来说，这可能看起来有点奇怪和笨拙，就像在你的情况下一样，但我相信你可以看到，当朝着另一个方向发展时，这是一种有用的行为，嵌套的属性代表了潜在的许多孩子中的一个或多个。
但是，根据post_attributes中的数据创建的post对象还没有与组相关联——post.group_id为nil。因此，当ActiveRecord进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为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会根据关联找到组，然后从结果中选择ID为1的组，成功，然后用group_attributes中的额外数据更新该组。
另外请注意，像您这样在帖子中包含组的嵌套属性的唯一原因是，如果您允许用户在创建新帖子的同时更新组名称。如果你想做的只是将新帖子链接到现有的组，那么你只需要在post_attributes中包含group_id，就可以去掉group_attribute。