MongoDB hash object projection

Question

If there is an object with unknown keys:

{
  data: {
    someObjectIdStringThatCantBePutInProjection: {
      dontReturn: 123,
      return: 321
    },
    someOtherObjectIdStringThatCantBePutInProjection: {
      dontReturn: 1234,
      return: 4321
    }
  }
}

And I want MongoDB to return only return property of the objects of the objects, what would the projection look like?

For example a projection

{
  data: { **allProperties**: { return: 1 } }
}

should return:

{
  data: {
    someObjectIdStringThatCantBePutInProjection: {
      return: 321
    },
    someOtherObjectIdStringThatCantBePutInProjection: {
      return: 4321
    }
  }
}

Answer 1

Using dynamic values as field names is considered an anti-pattern and introduces unnecessary complexity to queries. Nevertheless, you can convert the data object to an array of k-v tuples by $objectToArray. Use $map to get only the return field you need. Finally, use $arrayToObject to revert back to original form.

db.collection.aggregate([
  {
    "$set": {
      "data": {
        "$map": {
          "input": {
            "$objectToArray": "$data"
          },
          "as": "d",
          "in": {
            k: "$$d.k",
            v: {
              return: "$$d.v.return"
            }
          }
        }
      }
    }
  },
  {
    "$set": {
      "data": {
        "$arrayToObject": "$data"
      }
    }
  }
])

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