Custom resolver to existing schema property

MongoDB Atlas App Services & Realm Atlas GraphQL API
1

Is there any option available in Realm GraphQL to resolve the existing schema property?

{
	"title": "card",
	"properties": {
		"_id": {
			"bsonType": "objectId"
		},
		"filters": {
			"bsonType": "object",
			"properties": {
				"cardType": {
					"bsonType": "array",
					"title": "cardType",
					"items": {
						"bsonType": "object",
						"properties": {
							"cardName": {
								"bsonType": "string"
							}
						}
					}
				}
			}
		}
	}
}

In this example _id is the existing property of collection “card” but “filters” isn’t. I want to create a custom resolver for filters - cardType to get the unique list of cardType. It always returns null.

{
  "data": {
    "card": {
      "_id": "xyz",
      "filters": null
    }
  }
}

Please help to provide answer. My end objective is to add a few more filters to this.

Thanks in advance