Scala Play: json mapping (custom types)

// 1. Let's test custom types for Json serializer
import play.api.libs.json._

case class LastError(id: Option[Long] = None, userId: Long, error: Option[Exception] = None)

// add to routes: GET    /json/user/defined    controllers.TestController.jsonUserDefined
def jsonUserDefined = Action {
  val result = LastError(None, 55, Some(new RuntimeException("qwerty")))
  Ok(Json.toJson(result))
}

        // Compile output:
        // Error:(69, 19) No Json serializer found for type TestController.this.LastError.
        // Try to implement an implicit Writes or Format for this type.
        //  Ok(Json.toJson(result))

// 1.1. Now we add implicit serializer
case class LastError(id: Option[Long] = None, userId: Long, error: Option[Exception] = None)
implicit val lastErrToJson: Writes[LastError] = Json.writes[LastError]

        // Compile output:
        // Error:(65, 62) No instance of play.api.libs.json.Writes is available for scala.Option[java.lang.Exception]
        // in the implicit scope (Hint: if declared in the same file, make sure it's declared before)
        // implicit val lastErrToJson: Writes[LastError] = Json.writes[LastError]

// 1.3. so we need to provide extra implicit value to serialize Exception type
case class LastError(id: Option[Long] = None, userId: Long, error: Option[Exception] = None)
implicit val exceptionFormat: Format[Exception] = new Format[Exception] {
  def writes(t: Exception): JsValue = JsString(t.toString)
  def reads(json: JsValue): JsResult[Exception] = Json.fromJson[String](json) map {new Exception(_)}
}
implicit val lastErrToJson: Writes[LastError] = Json.writes[LastError]

        // Output:
        http://localhost:9000/json/user/defined
        {"userId":55,"error":"java.lang.RuntimeException: qwerty"}