ruby-on-rails - 获取每组最新的n条记录

Question

我有两个模型:话题评论

主题有_很多评论

如何获取每个主题的最后 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

你可以尝试使用last方法