如何在Mongoose中编写此聚合框架查询

Question

我必须编写一个按日期聚合和分组的查询。 示例文档就像这样

{
"_id" : ObjectId("567c02f0845d0abb468d99b4"),
"carId" : "345678",
"deviceId" : "073213",
"userId" : ObjectId("567a5e962c960d366fe7c989"),
"trips" : [
    {
        "startTime" : ISODate("2015-12-24T14:36:32.865Z"),
        "_id" : ObjectId("567c02f03223321f2b5581b8"),
        "tripON" : false,
        "source" : {
            "latitude" : 13.07,
            "longitude" : 77.6181319
        },
        "travelTime" : 0,
        "destination" : {
            "latitude" : 14.07,
            "longitude" : 78.6181319
        },
        "endTime" : ISODate("2015-12-24T14:36:57.416Z"),
        "topSpeed" : 11,
        "distance" : 231,
        "distanceUnit" : "km"
    },
    {
        "startTime" : ISODate("2015-12-24T14:37:08.466Z"),
        "_id" : ObjectId("567c03143223321f2b5581ba"),
        "tripON" : false,
        "source" : {
            "latitude" : 13.07,
            "longitude" : 77.6181319
        },
        "travelTime" : 0,
        "destination" : {
            "latitude" : 14.07,
            "longitude" : 78.6181319
        },
        "endTime" : ISODate("2015-12-24T14:37:54.471Z"),
        "topSpeed" : 11,
        "distance" : 231,
        "distanceUnit" : "km"
    },
    {
        "startTime" : ISODate("2015-12-24T19:16:34.771Z"),
        "_id" : ObjectId("567c44928242c35c4b169899"),
        "tripON" : false,
        "source" : {
            "latitude" : 13.07,
            "longitude" : 77.6181319
        },
        "travelTime" : 0,
        "destination" : {
            "latitude" : 14.07,
            "longitude" : 78.6181319
        },
        "endTime" : ISODate("2015-12-24T19:16:47.616Z"),
        "topSpeed" : 11,
        "distance" : 231,
        "distanceUnit" : "km"
    }
]

}

现在同一个用户可以有多辆车。并且查询在逻辑上评估为。查找特定用户的所有行程，无论是什么车，按日期隔离。 所以它就像

25-12-2015
Trip 1 
Trip 2
.
.
Trip n
24-12-2015 
Trip 1
Trip 2 
.
.
Trip n 

其中旅行n表示旅行的来源，目的地，旅行时间和距离。

我的尝试。

var result = Trip.aggregate([{
        "$unwind": "$trips"
    }, {
        "$match": {
            "userId" : userId,

            "trips.startTime": {
                "$gte": moment(startDate).toDate(),
                "$lte": moment(endDate).toDate()
            }
        }
    }, {
        "$group": {
            "_id": {
                "date": {
                    "$dayOfMonth": "$trips.startTime"
                }
            }

        }
    }]);

这会产生未定义的。我究竟做错了什么 ？怎么纠正呢？