longMapFormat.scala

implicit def longMapReads[T](implicit r: Reads[T]): Reads[Map[Long, T]] = Reads[Map[Long, T]] { jv =>
  jv.validate[Map[String, T]].flatMap { obj =>
    obj.foldLeft[JsResult[Map[Long,T]]](JsSuccess(Map.empty)) {
      case (JsSuccess(o, p), (k, v)) =>
        try {
          JsSuccess(o + (k.toLong -> v))
        } catch {
          case e: IllegalArgumentException =>
            val e = ValidationError(s"Value: '$k' cannot be converted to a Long")
            JsError(p, e)
        }
      case (err, (k, v)) =>
        // TODO: Accumulate errors...
        err
    }
  }
}

implicit def longMapWrites[T](implicit w: Writes[T]): Writes[Map[Long, T]] = Writes[Map[Long, T]] { map =>
  Json.toJson(map.map { case (k, v) =>
    k.toString -> Json.toJson(v)(w)
  })
}

implicit def longMapFormat[T](implicit r: Reads[T], w: Writes[T]): Format[Map[Long, T]] = Format(longMapReads,
  longMapWrites)