Question

post有likers和comments个孩子。我想根据它们对帖子进行排序。

class Post < ApplicationRecord
  scope :latest, -> {
    all.sort_by(&:ranking)
  }

  def ranking
    likers.count + comments.count
  end
end

这会调用以下查询：

  Post Load (0.7ms)  SELECT "posts".* FROM "posts"
   (0.4ms)  SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 52]]
   (0.4ms)  SELECT COUNT(*) FROM "users" INNER JOIN "user_post_likes" ON "users"."id" = "user_post_likes"."user_id" WHERE "user_post_likes"."post_id" = $1  [["post_id", 52]]
   (0.2ms)  SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 53]]
   (0.3ms)  SELECT COUNT(*) FROM "users" INNER JOIN "user_post_likes" ON "users"."id" = "user_post_likes"."user_id" WHERE "user_post_likes"."post_id" = $1  [["post_id", 53]]

所以我尝试以下方法：

Post.includes(:comments, :likers).all.sort_by(&:ranking)

这会调用以下查询：

  Post Load (0.7ms)  SELECT "posts".* FROM "posts"
  Comment Load (0.4ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN (52, 53, 54, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71)
  UserPostLike Load (0.3ms)  SELECT "user_post_likes".* FROM "user_post_likes" WHERE "user_post_likes"."post_id" IN (52, 53, 54, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71)
  User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 46
   (0.3ms)  SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 52]]
   (0.3ms)  SELECT COUNT(*) FROM "users" INNER JOIN "user_post_likes" ON "users"."id" = "user_post_likes"."user_id" WHERE "user_post_likes"."post_id" = $1  [["post_id", 52]]
   (0.2ms)  SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = $1  [["post_id", 53]]
   (0.3ms)  SELECT COUNT(*) FROM "users" INNER JOIN "user_post_likes" ON "users"."id" = "user_post_likes"."user_id" WHERE "user_post_likes"."post_id" = $1  [["post_id", 53]]

为什么会发生这种情况，我该如何解决？

更新

我想出了如何解决它，但是一个非常好的解释的答案会很好：

我必须将count替换为size。

初始

class Post < ApplicationRecord
  scope :latest, -> { 
    all.sort_by(&:ranking)
  }

  def ranking
    likers.count + comments.count
  end
end

后：

class Post < ApplicationRecord
  ...

  def ranking
    likers.size + comments.size
  end
end

然后，N+1 Query消失了。我得到的结论是，当你使用counter_cache时，会发生同样的事情。在这种情况下，我没有使用counter_cache，但我仍然必须使用size而不是count。我假设调用count强制Rails调用COUNT SQL查询并调用size使其使用内存中加载的记录。

Answer 1

你可以这样使用eager_load：

Post.eager_load(:comments, :likers).sort_by(&:ranking)

急切加载使用LEFT OUTER JOIN在单个查询中加载所有关联。

Eager Loading Associations

3 ways to do eager loading (preloading) in Rails 3 & 4

Answer 2

这里的问题有两个方面：

首先，sort_by立即为我举起一面旗帜： http://apidock.com/ruby/Array/sort%21

这是一个Array方法，意味着您不再构建ActiveRecord查询，而是进行数组转换。

由于您包含comments和likers，因此查询并不尽如人意，但这是另一个问题。

.count的工作方式是预制计数查询SELECT * FROM table。

要获得所需的结果，您需要构建自己的计数和排序查询。

看一下这篇文章，希望这会让你更好地了解如何进一步优化这个： Rails 3 ActiveRecord: Order by count on association

Answer 3

在您的情况下，最好的方法是使用counter_cache likers和comments。您可以阅读更多细节SHORT ARTICLE。这很简单，也是安全的时间和记忆。

如果您使用counter_cache，则不应向您的数据库发出多个请求。现在你的方法将是：

def ranking
  likers_count + comments_count
end

另一方面，如果您不想向表中添加列，请使用includes：

class Post < ApplicationRecord
  scope :latest, -> { 
    includes(:likers, :comments).sort_by(&:ranking)
  }

  def ranking 
    likers.count + comments.count
  end
end

但在这种情况下，每当方法调用时，您都会计算likers和comments

Rails避免N + 1查询

3 个答案: