json default field

json.scala

object JsonExtensions {
  def withDefaultCreatedAt[T](base: Format[T]) = new Format[T]{
    def reads(json: JsValue): JsResult[T] = base.compose(JsonExtensions.withDefault("createdAt", DateTime.now())).reads(json)
    def writes(o: T): JsValue = base.writes(o)
  }
  def withDefault[A](key:String, default:A)(implicit writes:Writes[A]) =
      __.json.update((__ \ key).json.copyFrom((__ \ key).json.pick orElse Reads.pure(Json.toJson(default))))
}

// .....
// usage 

implicit val userFmt = JsonExtensions.withDefaultCreatedAt(Json.format[User])