我试图获得MongoDB中集合的平均值和总和，为此我做了一个**$lookup**，它正确地返回信息，但当我想对它进行分组并获得总和和平均值时，这两个属性总是返回空值
这是我的MongoDB查询：

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 }
            }
        },
    ]
)

回应：

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

如何访问ventaporCliente.total？

• 谢谢-谢谢