Mongoose $ group＆amp; $ lookup aggregation返回$ unwind的空数组

Question

我有两个模式TravelRoute＆amp;机票。我试图找到Flight.fare.total_price的$ min并返回该结果，其中包含TravelRoute中的一些细节，这些细节在航班架构中被修改。我使用的是Mongo 3.4.1和Mongoose 4.8.5

const TravelRoute = new mongoose.Schema({
    departureAirport: {
        type: String,
        required: true,
        trim: true,
    },
    arrivalAirport: {
        type: String,
        required: true,
        trim: true,
    },
    durationDays: {
        type: Number
    },
    fromNow: {
        type: Number
    },
    isActive: {
        type: Boolean
    },
    class: {
        type: String,
        enum: ['economy', 'business', 'first', 'any']
    }
}

const Flights = new mongoose.Schema({
    route: {
        type: mongoose.Schema.Types.ObjectId, ref: 'TravelRoute'
    },
    departure_date: {
        type: Date
    },
    return_date: {
        type: Date
    },
    fare: {
        total_price: {
            type: Number
        }
    }
}

我有以下代码：

Flights.aggregate(
        {$group: {
                _id: {
                    route: '$route',
                    departure_date: '$departure_date',
                    return_date: '$return_date'
                },
                total_price: {$min: '$fare.total_price'}
            }
        },
        {$lookup: {
            from: 'TravelRoute',
            localField: '_id.route',
            foreignField: '_id',
            as: 'routes'
        }},
        {$unwind: '$routes'},
        {$project: {
            'routes.departureAirport': 1,
            'routes.destinationAirport': 1,
            'departure_date': 1,
            'return_date': 1,
            'total_price': 1,
            '_id': 1
        }},
        {$sort: {'total_price': 1}},
        function(err, cheapFlights){
            if (err) {
                log.error(err)
                return next(new errors.InvalidContentError(err.errors.name.message))
            }
            res.send(cheapFlights)
            next()
        })

当我使用$ unwind时，上面返回一个空数组[]。当我注释掉$ unwind阶段时，它会返回以下内容：

[
  {
    "_id": {
      "route": "58b30cac0efb1c7ebcd14e0a",
      "departure_date": "2017-04-03T00:00:00.000Z",
      "return_date": "2017-04-08T00:00:00.000Z"
    },
    "total_price": 385.6,
    "routes": []
  },
  {
    "_id": {
      "route": "58ae47ddc30b175150d94eef",
      "departure_date": "2017-04-03T00:00:00.000Z",
      "return_date": "2017-04-10T00:00:00.000Z"
    },
    "total_price": 823.68,
    "routes": []
  } 
...

我是Mongo和Mongoose的新手。令我感到困惑的是，即使我没有预测，它也会返回“路线”。即使我要求它也不会返回departure_date（etc）？我不确定我需要$ unwind因为每次飞行只有一个TravelRoute。

谢谢...

修改

这是最终的解决方案：

  {$lookup: {
            from: 'travelroutes', //<-- this is the collection name in the DB
            localField: '_id.route',
            foreignField: '_id',
            as: 'routes'
        }},
        {$unwind: '$routes'},
        {$project: {
            departureAirport: '$routes.departureAirport',
            arrivalAirport: '$routes.arrivalAirport',
            departureDate: '$_id.departure_date',
            returnDate: '$_id.return_date',
            'total_price': 1,
            'routeID': '$_id.route',
            '_id': 0
        }},
        {$sort: {'total_price': 1}},