Sort Parent by Count of Unique Values of a Child Field

[amarjotbhullar (Amarjot Singh Bhullar)]

Hi,
I am currently working on a feature where I have a parent child relationship between driver and trips taken by driver. I need to sort drivers by unique count of no. of cars driven irrespective of number of trips taken by driver.

Here is my mapping -

{
  "dynamic" : "false",
  "_source" : {
    "enabled" : true
  },
  "properties" : {
    "driver" : {
      "properties": {
        "driverId": {
          "type": "keyword"
        },
        "driverName": {
          "type": "text"
        },
        "driverDateOfBirth": {
          "type": "date"
        }
      }
    },
    "trips" : {
      "properties": {
        "tripId": {
          "type": "keyword"
        },
        "driverId": {
          "type": "keyword"
        },
        "carId": {
          "type": "text"
        },
        "tripDate": {
          "type": "date"
        }
      }
    },
    "my_join_field" : {
      "type" : "join",
      "relations" : {
        "driver" : [
          "trips"
        ]
      }
    }
  }
}

I need to sort driver by unique count of carId property of trips.

Now I understand we cannot directly sort parent by child and for my other sorts I am using function_score query with script_score to modify child score using child fields and using sum of all child scores as parent scores. But, for this scenario I do not have ideas as to how to solve the same.

Please help this innocent engineer you kindhearted people. Thanks!