使用 Lookup 通过 AVG MongoDB 获取平均值

Question

I am trying to obtain the average and sum of totals of a collection in MongoDB, for this I do a $lookup which returns the information correctly but when I want to group it and obtain the sum of totals as well as the average, those two properties always return them as null

This is my MongoDB Query:

db.clients.aggregate(
    [
        { 
            $lookup: { // DATA OK
                from: 'sales',
                localField: '_id',
                foreignField: 'clientId',
                as: 'ventaPorCliente'
            }
        },
        { 
            $group: { // total_average and sum null
                _id: "$idClient",
                username: { $first: "$name" },
                total_average: { $avg: 'ventaPorCliente.total'},
                sum: { $sum: 'ventaPorCliente.total'},
                count: { $sum: 1 }
            }
        },
    ]
)

Response:

[
  {
    "_id": "1",
    "username": "Peishion",
    "total_average": null,
    "sum": 0,
    "count": 1
  },
  {
    "_id": "1010",
    "username": "BENJAMIN",
    "total_average": null,
    "sum": 0,
    "count": 1
  }
]

How can i access to ventaporCliente.total ?

Thanks.

Answer 1

You missed the $ sign, also unwind the ventaPorCliente array, as $lookup pushes the matching objects in an array. Try this:

db.clients.aggregate(
    [
        { 
            $lookup: { 
                from: 'sales',
                localField: '_id',
                foreignField: 'clientId',
                as: 'ventaPorCliente'
            }
        },
        {
            $unwind: "$ventaPorCliente"
        }
        { 
            $group: { 
                _id: "$idClient",
                username: { $first: "$name" },
                total_average: { $avg: '$ventaPorCliente.total'},
                sum: { $sum: '$ventaPorCliente.total'},
                count: { $sum: 1 }
            }
        },
    ]
)