spray-json: JsonFormat for class hierarchy

TestJsonClassHierarchyFormat.scala

import spray.json._

object TestJsonClassHierarchyFormat extends App {
  sealed trait Animal extends Product
  case class Dog(name: String, age: Int) extends Animal
  case class Cat(name: String, catchesBirds: Boolean)extends Animal

  import DefaultJsonProtocol._

  implicit val dogFormat = jsonFormat2(Dog)
  implicit val catFormat = jsonFormat2(Cat)

  // format that discriminates based on an additional
  // field "type" that can either be "Cat" or "Dog"
  implicit val animalFormat = new RootJsonFormat[Animal] {
    def write(obj: Animal): JsValue =
      JsObject((obj match {
        case c: Cat => c.toJson
        case d: Dog => d.toJson
      }).asJsObject.fields + ("type" -> JsString(obj.productPrefix)))

    def read(json: JsValue): Animal =
      json.asJsObject.getFields("type") match {
        case Seq(JsString("Cat")) => json.convertTo[Cat]
        case Seq(JsString("Dog")) => json.convertTo[Dog]
      }
  }

  val a: Animal = Dog("Wuff", 23)
  println(a.toJson)
  println(a.toJson.convertTo[Animal])
  println("""{"type": "Dog", "name": "Wuff", "age": 23}""".parseJson.convertTo[Animal])
  println("""{"type": "Cat", "name": "Meow", "catchesBirds": true}""".parseJson.convertTo[Animal])
}