How can I get response with relationship after created and updated?

I want to show the relationship in response of JSON when created and updated.

Example like this

{
	"id": 1,
	"seat": 'R001',
	"room_id" 10,
	// I WANT TO BE SOMETHING LIKE THIS AFTER CREATED AND UPDATED
	"roomType": {
		"room_id": 10
		"content": "Bra Bra Bra"
	}
}

This is my model.

class SeatPosition extends Model {

  roomType() {
    return this.belongsTo('App/Models/ServiceRoom')
  }

}

And, This is creating codes.

        const seat = new SeatPosition()
        seat.position = position
        seat.service_room_id = service_room_id
        await seat.save()
        return seat

And, This is updating codes.

        const seat = await SeatPosition.find(id)
        seat.merge(
          {
            position,
            service_room_id
          }
        )
        await seat.save()
        return seat.position