case class SimpleQuery[T](field : String, op : Operator, value : T) extends Query{
  def getType = ????
}

implicit def SimpleQueryJsonFormat[A <: JsonFormat] = jsonFormat4(SimpleQuery.apply[A])

trait JsonFormat takes type parameters

case class NamedList[A](name: String, items: List[A])

object MyJsonProtocol extends DefaultJsonProtocol {
  implicit def namedListFormat[A :JsonFormat] = jsonFormat2(NamedList.apply[A])
}