Error_in_manual_SQL

==== Error

ActiveRecord::StatementInvalid (PGError: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ...comments ON comments.post_id = posts.id  ORDER BY comments.c...
                                                             ^
: SELECT distinct posts.* FROM "posts"  INNER JOIN comments ON comments.post_id = posts.id  ORDER BY comments.created_at DESC LIMIT 5):
  app/models/comment_activity.rb:18:in `find_recent'
  app/controllers/admin/dashboard_controller.rb:4:in `show'

===== Controller

class Admin::DashboardController < Admin::BaseController
  def show
    @posts            = Post.find_recent(:limit => 8)
    @comment_activity = CommentActivity.find_recent
    @stats            = Stats.new 
  end
end

===== Method

class CommentActivity
  attr_accessor :post

  def initialize(post)
    self.post = post
  end

  def comments
    @comments ||= post.approved_comments.find_recent(:limit => 5)
  end

  def most_recent_comment
    comments.first
  end

  class << self
    def find_recent
      Post.find(:all,
        :select => 'distinct posts.*',
        :joins  => 'INNER JOIN comments ON comments.post_id = posts.id',
        :order  => 'comments.created_at DESC',
        :limit  => 5
      ).collect {|post|
        CommentActivity.new(post)
      }.sort_by {|activity|
        activity.most_recent_comment.created_at
      }.reverse
    end
  end
end

Patch.rb

class << self
  def find_recent
    Post.find(:all,
      :select => 'distinct posts.*',
      :include => [:comments],
      :order  => 'comments.created_at DESC',
      :limit  => 5
    ).collect {|post|
      CommentActivity.new(post)
    }.sort_by {|activity|
      activity.most_recent_comment.created_at
    }.reverse
  end
end