小程序2.8.1和2.7.7 云数据库aggregeta有什么改动吗？

今天准备把之前的客服小程序升级成实时数据库监听新消息。

但一切换2.8.1。 之前的云数据库aggregate操作就失效了，返回了空数据

2.7.7

2.8.1

云数据库聚合代码

const $ = db.command.aggregate db.collection('msg').aggregate()      .group({        _id: "$customerOpenId",        "nickName": $.first("$nickName"),        "avatarUrl": $.first("$avatarUrl"),        "type": $.last("$type"),        "value": $.last("$value"),        "customerOpenId": $.last("$customerOpenId"),        "createTime": $.last("$createTime"),      })      .sort({        "createTime": -1      })      .end()      .then(res => {        let msg = res.list        if (msg.length) {          this.setData({            hasMsg: true          })        } else {          this.setData({            hasMsg: false          })        }        for (let item in msg) {          let date = new Date(msg[item].createTime)          let type = msg[item].type          switch (type) {            case 2:              msg[item].value = '图片消息';              break;            case 3:              msg[item].value = '语音消息';              break;          }          let sendTime = formatDate.formatTime(date)          msg[item].createTime = sendTime        }        this.setData({          msg: msg        })      })