MongoDB - $addFields get first object value to pass next object

I have a $addfields method. I need to get the assignedTo value of $addfields to next $lookup method.

I need to add some aggregation to get the cgPrice value. using $addfields but cgPrice not added in the response.

Existing Code:

let aggregate = aggregateInit ? aggregateInit : DatasetModel.aggregate();

aggregate = aggregate.addFields({
    state: getHistoryDateEntry("state", {}, "max"),
    lastFileId: getHistoryDateEntry(
        "fileId",
        {
            $ne: [{ $type: "$$item.fileId" }, "missing"],
        },
        "max"
    ),
    assignedTo: getHistoryDateEntry(
        "assignedTo",
        {
            $and: [
                { $ne: [{ $type: "$$item.assignedTo" }, "missing"] },
                {
                    $eq: ["$$item.state", State.AssignedToPost],
                },
            ],
        },
        "max"
    ),
    lastChange: getHistoryDateEntry("date", {}, "max"),
    firstAssigned: getHistoryDateEntry(
        "date",
        {
            $eq: ["$$item.state", State.AssignedToPost],
        },
        "min"
    ),
    firstBillable: getHistoryDateEntry(
        "date",
        {
            $in: [
                "$$item.state",
                [
                    State.Ready,
                    State.Error,
                    State.NotSymmetricalError,
                    State.AnnotationError,
                    State.FixAnnotationError,
                    State.VariantPositioningError,
                ],
            ],
        },
        "min"
    ),
    lastMeta: getHistoryDateEntry("meta", {}, "max"),
    lens: getHistoryDateEntry(
        "lens",
        {
            $eq: ["$$item.state", State.UpdateLens],
        },
        "max"
    ),
    lastStorageInfo: getHistoryDateEntry(
        "storageInfo",
        {
            $eq: ["$$item.state", State.UpdateStorageInfo],
        },
        "max"
    ),
    scanPositions: getHistoryDateEntry(
        "scanPositions",
        {
            $and: [
                { $ne: [{ $type: "$$item.scanPositions" }, "missing"] },
                {
                    $in: ["$$item.state", [State.Rescan, State.UpdateScanPositions]],
                },
            ],
        },
        "max"
    ),
    importWorkflow: {
        $cond: {
            if: {
                $ne: [
                    {
                        $type: getHistoryDateEntry(
                            "date",
                            {
                                $eq: ["$$item.state", State.Importing],
                            },
                            "max"
                        ),
                    },
                    "missing",
                ],
            },
            then: true,
            else: false,
        },
    },
    thumbnailUrl: getHistoryDateEntry(
        "fileId",
        {
            $eq: ["$$item.state", State.FinalQa],
        },
        "max"
    ),
});

my code:

aggregate = aggregate..lookup({
            from: "users",
            localField: "assignedTo",
            foreignField: "id",
            as: "matchedUsers",
        })
        .addFields({
            cgPrice: {
                $getField: {
                    field: "price",
                    input: {
                        $first: {
                            $filter: {
                                input: {
                                    $getField: {
                                        field: "prices",
                                        input: {
                                            $first: "$matchedUsers",
                                        },
                                    },
                                },
                                as: "currentPrice",
                                cond: {
                                    $and: [
                                        {
                                            $gte: ["$firstBillable", "$$currentPrice.beginDate"],
                                        },
                                        {
                                            $lt: ["$firstBillable", "$$currentPrice.endDate"],
                                        },
                                    ],
                                },
                            },
                        },
                    },
                },
            },
        })
        .project({
            matchedUsers: 0,
        })

but $lookup only support the string value. how to pass the assignedTo inside $lookup

Current Response data:

{
"state": "scanning",
    "lastFileId": "b4bfbab3-1f6f-4b83-bb15-f7bb8b1f1a0d",
    "assignedTo": "57xf16433c0d3ffe0ac084a4",
    "lastChange": "2020-05-05T07:29:01.639Z",
    "lastMeta": "Remark",
    "lens": {
      "coatingLabel": "Silver",
      "tintLabel": "Solid Grey 75%"
    },
    "lastStorageInfo": {
      "comment": "A15",
      "checkedOut": false
    },
    "scanPositions": 8,
    "checkedOut": false,
    "importWorkflow": false,
    "firstAssigned": "2020-05-05T07:29:01.639Z",
    "firstBillable": "2020-05-05T07:29:01.639Z",
    "thumbnailUrl": "https://www.url.com/test.png"
}

Excepted Response Data:

{
"state": "scanning",
    "lastFileId": "b4bfbab3-1f6f-4b83-bb15-f7bb8b1f1a0d",
    "assignedTo": "57xf16433c0d3ffe0ac084a4",
    "lastChange": "2020-05-05T07:29:01.639Z",
    "lastMeta": "Remark",
    "lens": {
      "coatingLabel": "Silver",
      "tintLabel": "Solid Grey 75%"
    },
    "lastStorageInfo": {
      "comment": "A15",
      "checkedOut": false
    },
    "scanPositions": 8,
    "checkedOut": false,
    "importWorkflow": false,
    "firstAssigned": "2020-05-05T07:29:01.639Z",
    "firstBillable": "2020-05-05T07:29:01.639Z",
    "cgPrice": 18.50,
    "thumbnailUrl": "https://www.url.com/test.png"
}

I want add cgPrice to current response.