Question

用户可以有很多帖子：

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

class Post < ActiveRecord::Base
  belongs_to :user
end

为什么以下序列不会更新第一篇文章？

$ rails c

> user = User.create(name: 'Misha')
 => #<User id: 7, name: "Misha", ... >
> user.posts << Post.create(description: 'hello')
 => #<ActiveRecord::Associations::CollectionProxy [#<Post id: 9, description: "hello", user_id: 7, ... >]> 
> post1 = Post.find(9)
> post1.assign_attributes(description: 'world')
> post1
 => #<Post id: 9, description: "world", user_id: 7, ... >
> post2 = Post.new(description: 'new post')
> user.posts = [post1, post2]
> user.posts.second.description
 => "new post"   # As expected
> user.posts.first.description
 => "hello"      # Why not "world"?

Answer 1

您正在混合保存帖子对象，并保存帖子与用户之间的关联。

像@zeantsoi所说，assign_attributes从不保存它 - 并且查看执行的SQL，collection=也没有保存任何内容。

> user.posts = [post1, post2]
   (0.1ms)  begin transaction
  SQL (0.7ms)  INSERT INTO "posts" ("created_at", "description", "updated_at", "user_id") VALUES (?, ?, ?, ?)  [["created_at", Mon, 17 Jun 2013 10:48:13 UTC +00:00], ["des
cription", "p2"], ["updated_at", Mon, 17 Jun 2013 10:48:13 UTC +00:00], ["user_id", 2]]
   (22.8ms)  commit transaction
=> [#<Post id: 3, description: "p1 modified", user_id: 2, created_at: "2013-06-17 10:46:43", updated_at: "2013-06-17 10:46:43">, #<Post id: 4, description: "p2", user_id: 
2, created_at: "2013-06-17 10:48:13", updated_at: "2013-06-17 10:48:13">]
>

只插入

post2因为必须设置关系才能插入;如果无法唯一地识别User，则Post对象无法知道特定的Post是否属于它。

查看构建CollectionAssociation的{{1}}的{{1}}来源observe how wholesale replacement is implemented：

has_many

工作的核心是replace_records：

# Replace this collection with +other_array+. This will perform a diff
# and delete/add only records that have changed.
def replace(other_array)
  other_array.each { |val| raise_on_type_mismatch!(val) }
  original_target = load_target.dup

  if owner.new_record?
    replace_records(other_array, original_target)
  else
    transaction { replace_records(other_array, original_target) }
  end
end

换句话说，它删除不在目标列表中的项目，然后添加不在新列表中的项目;结果是在收集分配期间根本没有触及目标和新列表（def replace_records(new_target, original_target) delete(target - new_target) unless concat(new_target - target) @target = original_target raise RecordNotSaved, "Failed to replace #{reflection.name} because one or more of the " \ "new records could not be saved." end target end）中的任何项目。

根据上面的代码，传递给参数的post1是返回的内容，似乎以反映更改：

target

但是在重新访问集合时，更改没有反映出来：

=> [#<Post id: 3, description: "p1 modified", user_id: 2, created_at: "2013-06-17 10:46:43", updated_at: "2013-06-17 10:46:43">, #<Post id: 4, description: "p2", user_id: 
2, created_at: "2013-06-17 10:48:13", updated_at: "2013-06-17 10:48:13">]

请注意，这里的回报略有不同;赋值的返回值是您传入的数组对象;这是> post1 => #<Post id: 3, description: "p1 modified", user_id: 2, created_at: "2013-06-17 10:46:43", updated_at: "2013-06-17 10:46:43"> > user.posts => #<ActiveRecord::Associations::CollectionProxy [#<Post id: 3, description: "p1", user_id: 2, created_at: "2013-06-17 10:46:43", updated_at: "2013-06-17 10:46:43">, #<Pos t id: 4, description: "p2", user_id: 2, created_at: "2013-06-17 10:48:13", updated_at: "2013-06-17 10:48:13">]> >。 reader function is called here：

ActiveRecord::Associations::CollectionProxy

然后，这将基于has_many关系创建集合代理，其值从我们分配选项时所知道的内容中填充。这个答案的唯一未被发现的部分就是生成的对象是清洗脏值 - 我已经做了一些代码阅读，并计算这将是最简单的一个调试器，这我没有心情回答对于。 :)但很明显它是从缓存中加载，或者传入的对象正在丢弃它们的更改。

无论哪种方式，如果您希望更改出现在目标对象中，您应该首先保存它 - 仅仅指定集合不够好，就好像它已经是成员一样，它不会被触及。

更新：有趣的是，请注意这只是因为我们使用# Implements the reader method, e.g. foo.items for Foo.has_many :items def reader(force_reload = false) if force_reload klass.uncached { reload } elsif stale_target? reload end @proxy ||= CollectionProxy.new(klass, self) end来获取Post.find;如果我们改为说post1，那么最后在post1 = (user.posts << Post.create(description: 'p1'))中观察到的集合实际上有脏对象。

这首先揭示了它是如何形成的。观看user.posts s：

object_id

请注意，集合代理中返回的对象与我们创建的对象相同。如果我们重新> u = User.create; p1 = (u.posts << Post.create(description: 'p1'))[0]; p1.assign_attributes(description: 'p1 mod'); p2 = Post.new(description: 'p2'); u.posts = [p1, p2]; u.posts ... => #<ActiveRecord::Associations::CollectionProxy [#<Post id: 21, description: "p1 mod", user_id: 10, created_at: "2013-06-17 11:43:30", updated_at: "2013-06-17 11:43:30">, #<Post id: 22, description: "p2", user_id: 10, created_at: "2013-06-17 11:43:30", updated_at: "2013-06-17 11:43:30">]> > _[0].object_id => 70160940234280 > p1.object_id => 70160940234280 >它：

find

让我困惑的原始问题的一部分是没有脏数据的对象来自哪里;没有SQL发生，甚至没有缓存命中，所以它必须来自某个地方。我原本以为它是其他缓存源，或者是明确地获取给定的对象并清理它们。

以上清楚地表明缓存实际上是我们在插入时创建的> u = User.create; u.posts << Post.create(description: 'p1'); p1 = Post.find(u.posts.first.id); p1.assign_attributes(description: 'p1 mod'); p2 = Post.new(description: 'p2'); u.posts = [p1, p2]; u.posts ...=> #<ActiveRecord::Associations::CollectionProxy [#<Post id: 23, description: "p1", user_id: 11, created_at: "2013-06-17 11:43:47", updated_at: "2013-06-17 11:43:47">, #<Post id: 24, description: "p2", user_id: 11, created_at: "2013-06-17 11:43:47", updated_at: "2013-06-17 11:43:47">]> > _[0].object_id => 70264436302820 > p1.object_id => 70264441827000 >。要100％确定，让我们看看返回的Post是否与创建的Post相同：

> u = User.create; p0 = (u.posts << Post.create(description: 'p1'))[0]; p1 = Post.find(u.posts.first.id); p1.assign_attributes(description: 'p1 mod'); p2 = Post.new(description: 'p2'); u.posts = [p1, p2]; u.posts
...
=> #<ActiveRecord::Associations::CollectionProxy [#<Post id: 27, description: "p1", user_id: 13, created_at: "2013-06-17 12:01:05", updated_at: "2013-06-17 12:01:05">, #<Post id: 28, description: "p2", user_id: 13, created_at: "2013-06-17 12:01:07", updated_at: "2013-06-17 12:01:07">]>
> _[0].object_id
=> 70306779571100
> p0.object_id
=> 70306779571100
> p1.object_id
=> 70306779727620
>

因此，CollectionProxy中不反映变化的对象实际上是我们在首先追加到集合时创建的对象;这解释了缓存数据的来源。然后我们置换副本，这不会在收集后分配后反映出来。

Rails：为什么“collection =”不会更新具有现有ID的记录？

1 个答案: