导轨ActiveRecord关联不一致更新

Question

我遇到了一些的Rails 2.3.5 ActiveRecord的行为，我不明白。看来，一个对象可以有它的关联标识不一致的方式更新。

此最好用一个例子说明：

创建与所述串的Post模型属性'title'以及与串的Comment模型属性'content'。

下面是关联：

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

方案1：在下面的代码创建一个Post与相关联的Comment，通过Post'ing第一创建第二find，添加第二Comment到第一Post并发现第二Post具有相关联的第二Comment到它没有显式分配。

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，在它的面，不应该影响的结果。额外的命令是post2.comments发生后创建comment2和前添加到comment2 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，这只是存在的与post2在这种情况下，而在方案1相关联的一个注释有两个。

大问题：添加新post2.comments到Comment任何区别，其评论用post1关联之前为什么会运行post2？

Answer 1

这具有与活动记录缓存请求和has_many关联的处理方式的方式做。

除非关联与eagerloaded：包括查找期间选项。直到需要Rails会不填充协会发现的记录。当需要的相关的一些记忆化做是为了减少对执行的SQL查询的数量。

通过在问题的代码步进：

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]

方案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在内部定义为post2.comments.map(&:id)

P.S。我的回答这个问题可能会帮助你理解为什么POST2得到尽管你不接触更新。