MongoDB AddField子数组的计算和

假设我有一个简单的对象，其子数组如下：

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "Installments": [
      {
        "InstallmentId": "Inst01",
        "TotalPaid": 2
      },
      {
        "InstallmentId": "Inst02",
        "TotalPaid": 5
      }
    ]
  }
]

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "TotalInstallments": 7,
    "TotalBalanceDue": 3,
  }
]

db.collection.aggregate([
  {
    $addFields: {
      "TotalInstallments": {
        $sum: "$Installments.TotalPaid"
      },
      "TotalBalanceDue": {
        $subtract: [
          "$TotalBilling",
          {
            $sum: "$Installments.TotalPaid"
          }
        ]
      }
    }
  }
])

我想运行一个聚合到$addified，它将是数组分期付款的'TotalPaid'的$sum。我会将结果添加到另一个字段中，这将是TotalBilling与TotalInstallations的美元减去。。。。会是这样的：

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "Installments": [
      {
        "InstallmentId": "Inst01",
        "TotalPaid": 2
      },
      {
        "InstallmentId": "Inst02",
        "TotalPaid": 5
      }
    ]
  }
]

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "TotalInstallments": 7,
    "TotalBalanceDue": 3,
  }
]

db.collection.aggregate([
  {
    $addFields: {
      "TotalInstallments": {
        $sum: "$Installments.TotalPaid"
      },
      "TotalBalanceDue": {
        $subtract: [
          "$TotalBilling",
          {
            $sum: "$Installments.TotalPaid"
          }
        ]
      }
    }
  }
])

我想知道我是否会使用Map并推送分期付款或减少或组的值

---

您可以使用如下聚合：

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "Installments": [
      {
        "InstallmentId": "Inst01",
        "TotalPaid": 2
      },
      {
        "InstallmentId": "Inst02",
        "TotalPaid": 5
      }
    ]
  }
]

[
  {
    "BillingId": 1,
    "TotalBilling": 10,
    "TotalInstallments": 7,
    "TotalBalanceDue": 3,
  }
]

db.collection.aggregate([
  {
    $addFields: {
      "TotalInstallments": {
        $sum: "$Installments.TotalPaid"
      },
      "TotalBalanceDue": {
        $subtract: [
          "$TotalBilling",
          {
            $sum: "$Installments.TotalPaid"
          }
        ]
      }
    }
  }
])

---

如果分期付款为NULL或空，如何默认为0（零）？，并将默认为0。另一个问题。。。当分期付款总额为字符串而不是十进制时。。。直接求和是行不通的。。。是否应该应用映射和用户$toDecimal，然后是$sum？刚刚添加了另一个名为Installments2的道具，并对分期付款进行映射。Total和Just$toDecimal。很好！谢谢