按时间戳排序的聚合排序不起作用

Question

我正在尝试使用我帖子中的用户名获取所有最新评论。我尝试使用以下方式获取最新记录：

"$sort": {
          "rd_comments.created_at": -1
        }

以下是完整的查询：

 [   {
     "$lookup": {
       "from": "userlist",
       "localField": "u_id",
       "foreignField": "u_id",
       "as": "userlist"
     }   },   {
     "$unwind": "$userlist"   },   {
     "$match": {
       "status": {
         "$ne": 2
       }
     }   },   {
     "$skip": 0   },   {
     "$limit": 10   },   {
     "$sort": {
       "rd_comments.created_at": -1
     }   },   {
     "$project": {
       "username": "$userlist.username",
       "_id": 1,
      "comment": 1,
       "created_at": 1
     }   } ]

它给了我未分类的结果。

[增订]

用户文档：

u_id    |   username
--------------------------
1       |   Shail
--------------------------
2       |   Mukky
--------------------------

评论文件：

c_id    |   u_id   |   comments  | created_at
--------------------------------------------------------
1       |   1      | Nice Post3  | NumberInt(1483405261)
--------------------------------------------------------
2       |   2      | Nice Post3  | NumberInt(1483318861)
--------------------------------------------------------
3       |   2      | Nice Post2  | NumberInt(1488522534)
--------------------------------------------------------
4       |   2      | Nice Post1  | NumberInt(1488522033)
--------------------------------------------------------
5       |   1      | Nice Post2  | NumberInt(1488521530)
--------------------------------------------------------