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Rails ActiveRecord关联更新不一致

belongs-to has-many associations activerecord ruby-on-rails

rlandster · 技术社区 · 16 年前

我遇到了一些我不理解的Rails 2.3.5活动记录行为。对象的关联ID可能以不一致的方式更新。

这最好用一个例子来解释:

创建一个 Post 具有字符串属性的模型 'title' 和A Comment 具有字符串属性的模型 'content' .

以下是关联:

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

场景1:在下面的代码中,我创建了一个 柱 与关联的 评论 ,创建第二个 柱 通过 find '在第一个,添加第二个 评论 到第一 柱 发现第二个 柱 有第二 评论 与它关联,但没有显式赋值。

post1 = Post.new
post1 = Post.new(:title => 'Post 1')
comment1 = Comment.new(:content => 'content 1')
post1.comments << comment1
post1.save
# Create a second Post object by find'ing the first
post2 = Post.find_by_title('Post 1')
# Add a new Comment to the first Post object
comment2 = Comment.new(:content => 'content 2')
post1.comments << comment2 
# Note that both Comments are associated with both Post objects even
# though I never explicitly associated it with post2.
post1.comment_ids # => [12, 13]
post2.comment_ids # => [12, 13]

场景2:再次运行上述命令,但这次插入一个额外的命令,从表面上看,该命令不应影响结果。额外的命令是 post2.comments 发生了什么之后创建 comment2 和之前添加 评论2 到 post1 .

post1 = Post.new
post1 = Post.new(:title => 'Post 1A')
comment1 = Comment.new(:content => 'content 1A')
post1.comments << comment1
post1.save
# Create a second Post object by find'ing the first
post2 = Post.find_by_title('Post 1A')
# Add a new Comment to the first Post object
comment2 = Comment.new(:content => 'content 2A')
post2.comments # !! THIS IS THE EXTRA COMMAND !!
post1.comments << comment2 
# Note that both Comments are associated with both Post objects even
# though I never explicitly associated it with post2.
post1.comment_ids # => [14, 15]
post2.comment_ids # => [14]

注意只有一与关联的注释 post2 在这个场景中,而在场景1中有两个。

大问题:为什么要跑步 帖子2评论 在添加新的 评论 到 POST 1 对注释所关联的内容进行任何更改 POST 2 ?

1 回复 | 直到 16 年前

Community Mohan Dere 9 年前

这与活动记录缓存请求的方式以及处理多个关联的方式有关。

除非该关联在查找过程中被急切地加载了:include选项。在需要之前,Rails不会填充找到的记录的关联。当需要关联时 memoization 这样做是为了减少执行的SQL查询的数量。

单步执行问题中的代码:

post1 = Post.new(:title => 'Post 1')
comment1 = Comment.new(:content => 'content 1')
post1.comments << comment1  # updates post1's internal comments cache
post1.save 

# Create a second Post object by find'ing the first
post2 = Post.find_by_title('Post 1') 

# Add a new Comment to the first Post object
comment2 = Comment.new(:content => 'content 2')
post1.comments << comment2   # updates post1's internal comments cache

# Note that both Comments are associated with both Post objects even
# though I never explicitly associated it with post2.
post1.comment_ids # => [12, 13]

# this is the first time post2.comments are loaded. 
# SELECT comments.* FROM comments JOIN comments.post_id = posts.id WHERE posts.id = #{post2.id}
post2.comment_ids # => [12, 13]

情景2:

post1 = Post.new(:title => 'Post 1A')
comment1 = Comment.new(:content => 'content 1A')
post1.comments << comment1
post1.save

# Create a second Post object by find'ing the first
post2 = Post.find_by_title('Post 1A')

# Add a new Comment to the first Post object
comment2 = Comment.new(:content => 'content 2A')

# first time post2.comments are loaded. 
# SELECT comments.* FROM comments JOIN comments.post_id = posts.id WHERE 
#   posts.id = post2.comments #=> Returns one comment (id = 14)
# cached internally.

post1.comments << comment2 
# Note that both Comments are associated with both Post objects even
# though I never explicitly associated it with post2.
post1.comment_ids # => [14, 15]

# post2.comment has already been cached, so the SQL query is not executed again.

post2.comment_ids # => [14]

N.B. post2.comment_ids 内部定义为 post2.comments.map(&:id)

P.S.我的回答 this question 也许可以帮助你理解为什么post2会被更新,尽管你没有接触它。