在MongoDB中执行$ lookup之后如何在$ project特定字段

Question

我有一个类似的查询

let issuerPaid = await ChallengeModel.aggregate([
    { $match: {voucher_paid_by: 'issuer', challenge_by_appuser: mongoose.Types.ObjectId(req.body.user_id)} },
    { $lookup: {from: 'vouchers',localField: 'voucher_id',foreignField: '_id',as: 'voucher'} },
    { $lookup: {from: 'appusers',localField: 'challenge_loser',foreignField: '_id',as: 'challenge_loser'} },
]); 

结果就像

{
    "issuerPaid": [
        {
            "_id": "5b86b18afa04445392ff6542",
            "voucher_id": "5b7cfa381496bea7d6c5bea4",
            "voucher_paid_by": "loser",
            "challenge_loser": [
                {
                    "_id": "5b75643c0a97791bcc9ed64c",
                    "status": "1",
                    "last_name": "Gmail",
                    "first_name": "Vishwas",
                    "created_at": "2018-08-16T11:47:08.475Z",
                    "updated_at": "2018-08-28T12:54:45.462Z"
                }
            ],
            "voucher": [
                {
                    "_id": "5b7cfa381496bea7d6c5bea4",
                    "voucher_name": "Walora",
                    "voucher_description": "Where fashion meets fitness! Get a 20% discount on all active wear purchases",
                    "voucher_disclaimer": "",
                    "voucher_validity": "2018-10-31 00:00:00.666",
                    "voucher_logo": "/vouchers/5b7cf73ccee4b5b7cf73ccee85.jpg",
                    "status": "1",
                    "created_at": "2018-08-22T05:52:56.156Z",
                    "updated_at": "2018-08-22T05:52:56.156Z"
                }
            ]
        }
    ]
}

但是它返回appusers的所有字段。但是我只想要appusers的{​​{1}}和first_name。

我尝试last_name和$unwind仅$project和first_name，但是当没有last_name时，它返回0条记录，而我想要返回记录（如果不存在）。因此我无法使用voucher_id。

我希望结果看起来像

$unwind

现在我在计划特定领域时遇到了麻烦。

请帮助。