Play Json Map[K,V]

map.scala

   /** Play Json only supports Map[String,V]. This function creates a format for Map[K,V]. The type K should produce a JsString.
    * Otherwise the serialisation will fail. Which is good enough since in valid json keys can only be strings. */
 def mapFormat[K, V](implicit fk: Format[K], fv: Format[V]): Format[Map[K, V]] = new OFormat[Map[K, V]] {
    override def writes(o: Map[K, V]): JsObject = {
      val stringMap = o.map { case (k, v) => (Json.toJson[K](k).as[JsString].value, v) }
      Json.toJson(stringMap).as[JsObject]
    }

    override def reads(json: JsValue): JsResult[Map[K, V]] = {
      for {
        stringMap <- Json.fromJson[Map[String, V]](json)
        _         <- Json.fromJson[Set[K]](Json.toJson(stringMap.keySet))
      } yield stringMap.map { case (k, v) => (Json.fromJson[K](JsString(k)).get, v) }
    }
  }