sql – ActiveRecord：使用组计数

我有一个带有has_many评论的帖子模型.每个帖子都有一个线程ID(但没有名为Thread的模型).

所以,如果我想计算这篇文章中的线程数,我就会喜欢

> post.comments.count(:thread,:distinct => true)
SELECT COUNT(disTINCT "comments"."thread") FROM "comments" WHERE "comments"."post_id" = 3

这很好用.但是,如果我只想用一条评论计算线程数呢？

> post.comments.group(:thread).having('COUNT(*) == 1').count
SELECT COUNT(*) AS count_all,thread AS thread FROM "comments" WHERE "comments"."post_id" = 3 GROUP BY thread HAVING COUNT(*) == 1 ORDER BY id

所以我有一个OrderedHash而不是Integer.我必须做不必要的步骤

> post.comments.group(:thread).having('COUNT(*) == 1').count.count

有没有更好的解决方案？

解决方法

这是预期的行为.

post.comments.group(:thread).having('COUNT(*) == 1').count

这将返回一个散列,其中键是线程ID,值是计数.我相信只要你在rails中使用聚合函数组成一个组就是这种情况.您必须执行第二次计数以获取第一个查询匹配的结果数.

我不确定Rails中会有什么样子,但这是我认为你想要的sql：

SELECT COUNT(*) FROM 
  SELECT COUNT(*) AS count_all,thread AS thread 
  FROM "comments"
  WHERE "comments"."post_id" = 3 
  GROUP BY thread
  HAVING COUNT(*) == 1
  ORDER BY id