How to lookup a specific field of every object of an array using mongodb aggregation

I'm trying to lookup every rentalContractId from rentalcontractdetails table. The product document looks something like this:

{
  "productPrice": [
    {
      "rentalContractId": ObjectId("64018dfc8a7d37f378d6c427"),
      "price": 20
    },
    {
      "rentalContractId": ObjectId("64018dfc8a7d37f378d6c426"),
      "price": 10
    }
  ]
}

I just need to lookup rentalContractId. So was doing the below query in aggregation to get that output,

db.products.aggregate([
{
      $lookup: {
        from: "rentalcontractdetails",
        localField: "productPrice.rentalContractId",
        foreignField: "_id",
        as: "rentalContract",
      },
    }, { $set: {
      "productPrice.rentalContract": "$rentalContract"
    }}])

but this is producing output like this.
"productPrice":

[
                    {
                        "rentalContractId": "64018dfc8a7d37f378d6c427",
                        "price": 20,
                        "rentalContract": [
                            {
                                "_id": "64018dfc8a7d37f378d6c427",
                                "contractType": "8 - Days Rental",
                                "contractDaysInNumber": 8,
                                "createdAt": "2023-03-03T06:04:44.531Z",
                                "isDeleted": false,
                                "isArchived": false
                            },
                            {
                                "_id": "64018dfc8a7d37f378d6c426",
                                "contractType": "4 - Days Rental",
                                "contractDaysInNumber": 4,
                                "createdAt": "2023-03-03T06:04:44.531Z",
                                "isDeleted": false,
                                "isArchived": false
                            }
                        ]
                    },
                    {
                        "rentalContractId": "64018dfc8a7d37f378d6c426",
                        "price": 10,
                        "rentalContract": [
                            {
                                "_id": "64018dfc8a7d37f378d6c427",
                                "contractType": "8 - Days Rental",
                                "contractDaysInNumber": 8,
                                "createdAt": "2023-03-03T06:04:44.531Z",
                                "isDeleted": false,
                                "isArchived": false
                            },
                            {
                                "_id": "64018dfc8a7d37f378d6c426",
                                "contractType": "4 - Days Rental",
                                "contractDaysInNumber": 4,
                                "createdAt": "2023-03-03T06:04:44.531Z",
                                "isDeleted": false,
                                "isArchived": false
                            }
                        ]
                    }
                ]

expected output:

{
  "productPrice": [
    {
      "rentalContractId": {
          "_id": "64018dfc8a7d37f378d6c427",
          "contractType": "8 - Days Rental",
        },
      "price": 20
    },
    {
      "rentalContractId": {
          "_id": "64018dfc8a7d37f378d6c426",
          "contractType": "4 - Days Rental",
        }
      "price": 10
    }
  ]
}

Thanks in advance.

One option is to use $lookup with pipeline, and update your $set step to use $mergeObjects inside $map:

db.products.aggregate([
  {$lookup: {
      from: "rentalcontractdetails",
      localField: "productPrice.rentalContractId",
      foreignField: "_id",
      as: "rentalContract",
      pipeline: [{$project: {contractType: 1}}]
  }},
  {$project: {
      _id: 0,
      productPrice: {$map: {
          input: "$rentalContract",
          in: {$mergeObjects: [
              {rentalContractId: "$$this"},
              {price: {$arrayElemAt: [
                    "$productPrice.price",
                    {$indexOfArray: ["$productPrice.rentalContractId", "$$this._id"]}
              ]}}
          ]}
      }}
  }}
])

See how it works on the playground example

• mongodb $lookup and mongoose populate are two different things, working with two different mechanisms. The original question was mixing the two, but used $lookup in the code, so I guess this was the intention.