获取每个组的最新n条记录

Question

我有两个型号： 话题 评论

主题has_many评论

我如何才能获得每个主题的最后4条评论？

我试过这个但是很慢：

Comment.
      joins("INNER JOIN (SELECT m1.id,m1.topic_id, COUNT(m2.id) as c FROM multimedias as m1 LEFT JOin multimedias as m2 ON 
        m1.topic_id = m2.topic_id  AND m1.id < m2.id 
        WHERE m1.topic_id IS NOT NULL GROUP BY m1.id, m1.topic_id HAVING c < 4
        ORDER by m1.topic_id, c desc) as m3 ON m3.id = multimedias.id").
      where([ "multimedias.topic_id IN (?)", topics_id ]) 

Answer 1

您可以尝试使用last方法

Answer 2

#last方法将整数作为参数，因此您只需使用Comment.last(4)

Answer 3

的ActiveRecord :: FinderMethods＃尾

#last有一个可选参数，允许您指定最后N条记录，如下所示：

Comment.
      joins("INNER JOIN (SELECT m1.id,m1.topic_id, COUNT(m2.id) as c FROM multimedias as m1 LEFT JOin multimedias as m2 ON 
        m1.topic_id = m2.topic_id  AND m1.id < m2.id 
        WHERE m1.topic_id IS NOT NULL GROUP BY m1.id, m1.topic_id HAVING c < 4
        ORDER by m1.topic_id, c desc) as m3 ON m3.id = multimedias.id").
      where([ "multimedias.topic_id IN (?)", topics_id ]).
      last(4)