Mongodb 如何拉平mongo DB中的对象并在根目录中保存时有条件地重命名字段

我有一个集合，其中包含需要拉平的平衡对象，并且该对象中的字段需要根据集合中的另一个标志重新命名-

[
  {
    "id": "1234",
    "documentType": "IV",
    "balance": {
      "total": 100,
      "openBalance": 60,
      "paid": 40
    }
  },  
  {
    "id": "9012",
    "documentType": "CM",
    "balance": {
      "total": 50,
      "applied": 0,
      "available": 50
    }
  }
]

如果documentType =="IV"，那么"balance. payed"将成为$$ROOT中的"totalAmountPaid"，而如果是"CM"，那么"balance.applied"将重命名为appliedAmount，将"balance. availableAmount"重新命名为availableAmount，因此压平余额对象后的最终收款变成-

[
  {
    "id": "1234",
    "documentType": "IV",
    "total": 100,
    "openBalance": 60,
    "totalAmountPaid": 40
  },  
  {
    "id": "9012",
    "documentType": "CM",
    "total": 50,
    "appliedAmount": 0,
    "availableAmount": 50
  }
]

我try 像这样使用$set和$cond，但它不起作用，而且我对MongoDB命令也不是很熟悉-

db.collection.aggregate([
  {
    $set: {
      $cond: {
        if: {
          $eq: [
            "$documentType",
            "IV"
          ]
        },
        then: {
          "total": "$balance.total",
          "openBalance": "$balance.openBalance",
          "totalAmountPaid": "$balance.paid",
          "balance": "$$REMOVE"
        },
        else: {
          "$total": "$balance.total",
          "$availableAmount": "$balance.available",
          "$appliedAmount": "$balance.applied",
          "balance": "$$REMOVE"
        }
      }
    }
  }
])