课程讨论区动态按最后更新时间排序

app/controllers/boards_controller.rb

@@ -137,7 +137,7 @@ class BoardsController < ApplicationController
         #end
         @topic_count = @board.topics.count();
         @topic_pages = (params[:page] ? params[:page].to_i + 1 : 0) *10
-        @topics = @board.topics.reorder("#{Message.table_name}.sticky DESC, #{Message.table_name}.created_on desc").
+        @topics = @board.topics.reorder("#{Message.table_name}.sticky DESC, COALESCE(last_replies_messages.created_on, #{Message.table_name}.created_on) desc").
             limit(limit).offset(@topic_pages).includes(:last_reply).
             preload(:author, {:last_reply => :author}).all();
       else