轨道3：。集团（）下令created_at [英] Rails 3: .group() ordered by created_at

问题描述

什么是Rails 3的方式订购。集团（）导致ActiveRecord的（这里的created_at）？

What is the Rails 3 way to order .group() results in Activerecord (here by "created_at")?

@messages = Message.group(:foo)

只会导致显示最早的消息。我需要最新的中显示。

only results in displaying the oldest message. I'd need the latest to be shown.

我试过

@messages = Message.group(:foo).having("created_at = MAX(created_at)")

没有成功。任何提示AP preciated！

with no success. Any hints appreciated!

要澄清：我希望有自己内部的组有序的，不正常的messages.order（...）。 如果有不容易的ActiveRecord语法，我会很乐意与原始SQL以及

To clarify: I'm looking to have the group ordered within itself, not a normal messages.order("..."). Should there be no easy Activerecord syntax, i'd be happy with raw SQL as well

更新：尝试在SQL的方式，这是这样运作的：

Update: trying the SQL way, this was supposed to work:

@messages = Message.find_by_sql("
  SELECT messages.* 
  FROM messages 
  GROUP BY messages.foo 
  HAVING messages.created_at = MAX(messages.created_at) 
  ORDER BY messages.created_at DESC")

但是，这仅检索单记录（那些不分组）。据说分组那些被忽略。不知道为什么，所有的记录有：created_at和：FOO值

But this retrieves solely single records (those that are not grouped). Supposedly grouped ones are being omitted. Do not know why, all records have :created_at and :foo values

推荐答案

心动不如行动通过使用多个专用子查询工作，没有GROUP BY电话：

Got it all working by using a more dedicated subquery, without a GROUP BY call:

SELECT *
   FROM `messages`
   WHERE `id` = (
      SELECT `id`
      FROM `messages` as `alt`
      WHERE `alt`.`foo` = `messages`.`foo`
      ORDER BY `created_at` DESC
      LIMIT 1
   )
   ORDER BY `created_at` DESC

感谢所有在这个线程混乱的回答： MySQL的集团通过订购

不知道有任何速度的影响尚未虽然。如果有人想介入，请随时这样做。

Don't know about any speed implications yet though. If anyone wants to step in, please feel free to do so.

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