sql查询顺序问题

Question

因此，我在 activerecord 中有以下模型，我将使用 Discussion.user_replied_group_discussions(@user) 它会返回最近回复的讨论，但是如何修改私有方法以按以下方式对返回的讨论进行排序最晚回复时间？

# Table name: discussions
#
#  id               :integer         not null, primary key
#  title            :string(255)     not null
#  content          :text(255)       not null
#  created_at       :datetime
#  updated_at       :datetime
#  user_id          :integer
#  discussable_id   :integer
#  discussable_type :string(255)
class Discussion < ActiveRecord::Base
  has_many    :comments, :as => :commentable, :dependent => :destroy
  #
  #this is the scope that I am having trouble with:
  #
  scope :user_replied_group_discussions, lambda {|user| replied_by(user) }

  private
    def self.replied_by(user)
      discussion_ids = %(SELECT commentable_id FROM comments 
                     WHERE commentable_type = 'Discussion' AND user_id = :user_id)
      where("discussable_type = 'Group' AND id IN (#{discussion_ids}) ", 
           { :user_id => user })
    end
end

# Table name: comments
#
#  id               :integer         not null, primary key
#  content          :text
#  created_at       :datetime
#  updated_at       :datetime
#  commentable_id   :integer
#  commentable_type :string(255)
#  user_id          :integer
#

class Comment < ActiveRecord::Base

  belongs_to :commentable, :polymorphic => true
  belongs_to :user

end

---

更新： @ypercube 提供的 sql 查询有所帮助，我现在将其与 find_by_sql 一起使用。但将其放入控制器中似乎很尴尬。

有更好的解决方案吗？谢谢！

Answer 1

这将根据最新的 discussions.updated_at 时间排序：

where("discussable_type = 'Group' AND id IN (#{discussion_ids}) 
order by updated_at DESC ",

如果您想按 comment.created_at 排序，请使用以下查询：

SELECT d.*
     , MAX(c.created_at) AS lastCommentTime
FROM discussions d
  JOIN comments c
    ON d.id = c.commentable_id
WHERE c.commentable_type = 'Discussion'
  AND c.user_id = userIDtoCheck
GROUP BY d.id
ORDER BY lastCommentTime DESC        <--- latest first