Rails的ActiveRecord關聯不一致更新

Question

我遇到了一些的Rails 2.3.5 ActiveRecord的行爲，我不明白。看起來一個對象可以以不一致的方式更新其關聯ID。

這最好用一個例子說明：

創建Post模型與串屬性'title'和Comment模型與串屬性'content'。

這裏有關聯：

class Post < ActiveRecord::Base 
    has_many :comments 
end 

class Comment < ActiveRecord::Base 
    belongs_to :post 
end 

場景＃1：在下面的代碼創建一個Post與相關聯的Comment，通過find創建第二Post「荷蘭國際集團的第一，添加第二Comment到第一Post並發現第二Post有沒有明確的分配與之相關的第二Comment。

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] 

場景＃2：再次運行上述命令，但這次插入一個額外的命令，在它的表面上，不應該影響結果。額外的命令是post2.comments這發生在之後創建comment2和之前添加comment2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，只有一個與post2在這種情況下，而在方案1相關的評論有兩個。

大問題：爲什麼會運行post2.comments添加新Comment到post1之前對這些意見進行了與post2相關的有什麼區別？

Answer 1

這與該活動記錄緩存請求和has_many關聯的處理方式的方式做。

除非協會與eagerloaded：包括查找過程中選擇。直到需要時，Rails纔會爲找到的記錄填充關聯。當需要關聯時，會執行一些memoization以減少執行的SQL查詢的數量。

通過在問題的代碼步進：

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在內部定義爲post2.comments.map(&:id)

P.S.我對this question的回答可能會幫助您理解爲什麼post2在您未觸及的情況下會更新。