Definining a Generic JsonFormat. If SequentialId weren't a Generic, this would be easy. Since it is, and I want to serialize it directly to a String (not as a case-class containing an "id" field), I'm having trouble...

Model.scala

import spray.json._

class Model {
  implicit object SequentialIdJsonFormat extends JsonFormat[SequentialId] {
    def write(x: SequentialId) = JsString(x.id)
    def read[T](value:JsValue):SequentialId = value match {
      case JsString(x) => SequentialId[T](x)
      case x => deserializationError("Expected SequentialId as JsString, but got " + x)
    }
  }
}