order by id instead of created_at

id is indexed while created_at is not

app/views/forums/index.html.erb

@@ -18,7 +18,7 @@
               <%= link_to f.name, f, id: "forum-#{f.id}"%>
               <div class="item-info">
                 <% if last_thread = f.threads.last %>
-                  <% last_reply = Threadreply.where(forumthread: f.threads).order(:created_at).last %>
+                  <% last_reply = Threadreply.where(forumthread: f.threads).order(:id).last %>
                   <% if last_reply && last_reply.created_at > last_thread.created_at %>
                     <% if last_reply.thread.can_read?(current_user) %>
                       <%= last_reply.author.name %>