MongoDB create nested top-down hierarchy using aggregation

I am trying to create a nested aggregation on a single collection in MongoDB using $graphLookup with keys dependentOn and usedBy which are of Object datatype that store array of categories and the _id field inside.

Input collection sample (relations):

// Doc 1:

{
  "_id": "p_commerce_metrics_dtst_v1",
  "category": "dataset",
  "dependentOn": {
    "metrics": [
      "net_booking_count_v1"
    ]
}

// Doc 2:

{
  "_id": "net_booking_count_v1",
  "category": "metric",
  "dependentOn": {
    "metrics": [
      "cancelled_booking_count_v1",
      "gross_booking_count_v1"
    ]
  }
}

// Doc 3:

{
  "_id": "cancelled_booking_count_v1",
  "category": "metric",
  "dependentOn": {
    "measures": [
      "hb_cancel_measure_v1"
    ]
  }
}

// Doc 4:

{
  "_id": "gross_booking_count_v1",
  "category": "metric",
  "dependentOn": {
    "measures": [
      "hb_booking_measure_v1"
    ]
  }
}

// Doc 5 (Not dependentOn any other document _id. Dead End):

{
  "_id": "hb_cancel_measure_v1",
  "category": "measure",
  "usedBy": {
    "metrics": [
      "cancelled_booking_count_v1",
      "more_metrics"
    ]
  }
}

// Doc 6 (Not dependentOn any other document _id. Dead End):

{
  "_id": "hb_booking_measure_v1",
  "category": "measure",
  "usedBy": {
    "metrics": [
      "gross_booking_count_v1",
      "more_metrics"
    ]
  }
}

Expected Output:

// Fetch entire dependentOn hierarchy for p_commerce_metrics_dtst_v1

[
  {
    "name_version": "p_commerce_metrics_dtst_v1",
    "category": "dataset",
    "dependent_on": {
      "metrics": [
        "net_booking_count_v1",
        "cancelled_booking_count_v1",
        "gross_booking_count_v1"
      ],
      "measures": [
        "hb_cancel_measure_v1",
        "hb_booking_measure_v1"
      ]
    }
  }
]

I tried using the following aggregation query, but not able to get the desired output.

[
    { $match: {
        _id: "p_commerce_metrics_dtst_v1"
    }},
    { $graphLookup: {
        from: "relations",
        startWith: "$dependentOn",
        connectFromField: "dependentOn",
        connectToField: "_id",
        depthField: "depth",
        as: "dependentOn"
    }}
]

How can I achieve the desired output using aggregation query?

Your $graphLookup overrode the original value of dependentOn. Just use another name for the lookup result and unset it later. The remaining is just some data wrangling for flattening the lookup result.

db.relations.aggregate([
  {
    $match: {
      _id: "p_commerce_metrics_dtst_v1"
    }
  },
  {
    $graphLookup: {
      from: "relations",
      startWith: "$dependentOn.metrics",
      connectFromField: "dependentOn.metrics",
      connectToField: "_id",
      depthField: "depth",
      as: "dependentOnMetrics"
    }
  },
  {
    "$set": {
      "dependentOn": {
        "metrics": {
          "$setUnion": [
            {
              "$ifNull": [
                "$dependentOn.metrics",
                []
              ]
            },
            {
              "$reduce": {
                "input": "$dependentOnMetrics.dependentOn.metrics",
                "initialValue": [],
                "in": {
                  "$setUnion": [
                    "$$value",
                    "$$this"
                  ]
                }
              }
            }
          ]
        },
        "measures": {
          "$setUnion": [
            {
              $ifNull: [
                "$dependentOn.measures",
                []
              ]
            },
            {
              "$reduce": {
                "input": "$dependentOnMetrics.dependentOn.measures",
                "initialValue": [],
                "in": {
                  "$setUnion": [
                    "$$value",
                    "$$this"
                  ]
                }
              }
            }
          ]
        }
      }
    }
  },
  {
    "$unset": "dependentOnMetrics"
  }
])

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