没有字段类型的mongodb查询

Question

我是这里的新手，也是mongodb的新手。我有如下查询：

db.getCollection("devicepositions").aggregate([
        { $match: { portal: new ObjectId("5ecade2d8fdbb1046873af15")}},
        { $group: { _id: "$SerialNumber", ts: { $max: "$Timestamp"} }},
        { $lookup:
         {
           from: "devicepositions",
           let: { sn: "$_id", ts: "$ts"},
           pipeline: [
                { $match:
                    { $expr:
                        { $and:
                            [
                               { $eq: [ "$SerialNumber", "$$sn"] },
                               { $eq: [ "$Timestamp", "$$ts" ] }
                            ]
                        }
                    }
                },
                { $project: {Latitude:1, Longitude:1, Accuracy:1}}
            ],
            as: "position"
            }
     }
    ])

结果类似于：

{ 
    "_id" : "4343223323205", 
    "ts" : ISODate("2020-06-02T19:56:13.353+0000"), 
    "position" : [
        {
            "_id" : ObjectId("5ed79b0350869a5738e6a5ac"), 
            "Latitude" : NumberDecimal("50.9139106"), 
            "Longitude" : NumberDecimal("10.420745"), 
            "Accuracy" : NumberInt(1770)
        }
    ]
}

我如何从结果中删除字段类型（NumberInt / ObjectId等...） 预期输出应为：

{ 
    "_id" : "867688036663205", 
    "ts" : "2020-06-02T19:56:13.353+0000", 
    "position" : [
        {
            "_id" : "5ed79b0350869a5738e6a5ac", 
            "Latitude" : 50.9139106, 
            "Longitude" : 10.420745, 
            "Accuracy" : 1770)
        }
    ]
}

请让我朝正确的方向

谢谢