猫鼬-参数必须是集合管道运算符？

Question

尝试运行我的代码以从mongodb检索消息时，出现错误“参数必须是聚合管道运算符”。我试图以不同的方式构造我的代码，但不确定为什么这行不通。任何帮助将不胜感激，谢谢！

function(callback) {
  Message.aggregate([{
      $match: {
        $or: [{
            "senderName": req.user.username
          },
          {
            "receiverName": req.user.username
          }
        ]
      }
    },
    {
      $sort: {
        "createdAt": -1
      }
    },
    {
      $group: {
        "_id": {
          "last_message_between": {
            $cond: [{
                $gt: [{
                    $substr: ["$senderName", 0, 1]
                  },
                  {
                    $substr: ["$receiverName", 0, 1]
                  }
                ]
              },
              {
                $concat: ["$senderName", "and", "$receiverName"]
              },
              {
                $concat: ["$receiverName", "and", "$senderName"]
              }
            ]
          }
        },
        "body": {
          $first: "$$ROOT"
        }
      }
    },
    function(err, newResult) {
      callback(err, newResult);
    }
  ])
 }