const $ = db.command.aggregate

else if (event.a == 5) { //使用

    try {

      return await db.collection('chatmsg').aggregate().match({

        fromopenId: event.fromopenId,

      }).orderBy('time','desc').group({    //根据时间降序，取最近100用户数据

        _id: {  

          toopenId: '$toopenId',

          toavatar: '$toavatar',

          tonickName: '$tonickName',

          tosex: '$tosex',     

        },   

        readstate: $.sum('$readstate'),

        time: $.last('$time'), 

        content: $.last('$content'), 

        }).end()

    } catch (e) {

      console.log(e)

    }

  }