Rails ActiveRecord 关联更新不一致

Question

我遇到了一些我不理解的 Rails 2.3.5 ActiveRecord 行为。似乎一个对象可以以不一致的方式更新其关联 ID。

最好用一个例子来解释：

创建具有字符串属性'title' 的Post 模型和具有字符串属性'content' 的Comment 模型。

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Post 和一个关联的Comment，通过Post 创建第二个Post，将第一个Comment 添加到第一个@987654331 @ 并发现第二个 Post 与第二个 Comment 相关联，但没有明确分配。

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，应该不会影响结果。额外的命令是post2.comments，它发生在之后创建comment2和之前将comment2添加到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]

请注意，在这种情况下，只有 一个 评论与 post2 相关联，而在场景 1 中则有两个。

最大的问题：为什么在将新的 Comment 添加到 post1 之前运行 post2.comments 会对与 post2 关联的评论产生任何影响？

Answer 1

这与 Active Record 缓存请求的方式以及处理 has_many 关联的方式有关。

除非在查找过程中使用 :include 选项预先加载关联。在需要之前，Rails 不会填充找到的记录的关联。当需要关联时，会执行一些memoization 以减少执行的 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]

场景 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]

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

附：我对this question 的回答可能会帮助您理解为什么 post2 会在您没有触碰它的情况下得到更新。