Play JSON formatter for Map[Int,_]

Question

I am attempting to migrate a Rails/Mongodb application to Play 2.3 using play-reactivemongo and reactivemongo-extensions. In modeling my data I am running across a problem serializing and deserializing a Map[Int,Boolean].

When I try to define my formats via macro like so

implicit val myCaseClass = Json.format[MyCaseClass]

where MyCaseClass has a few string fields, a BSONObjectID field, and a Map[Int,Boolean] field the compiler complains with:

No Json serializer found for type Map[Int,Boolean]. Try to implement an implicit Writes or Format for this type.
No Json deserializer found for type Map[Int,Boolean]. Try to implement an implicit Reads or Format for this type.

Looking at the source code for Play in Reads.scala I see a Reads defined for Map[String,_] but none for Map[Int,_].

Is there a reason why Play has default Read/Writes for string maps but not for other simple types?

I don't fully understand the Map[String,_] defined by play because I am fairly new to scala. How would I go about translating that into a Map[Int,_]? If that is not possible for some technical reason how would I define a Reads/Writes for Map[Int,Boolean]?

Answer 1

you can write your own reads and writes in play.

in your case, this would look like this:

implicit val mapReads: Reads[Map[Int, Boolean]] = new Reads[Map[Int, Boolean]] {
    def reads(jv: JsValue): JsResult[Map[Int, Boolean]] =
        JsSuccess(jv.as[Map[String, Boolean]].map{case (k, v) =>
            Integer.parseInt(k) -> v .asInstanceOf[Boolean]
        })
}

implicit val mapWrites: Writes[Map[Int, Boolean]] = new Writes[Map[Int, Boolean]] {
    def writes(map: Map[Int, Boolean]): JsValue =
        Json.obj(map.map{case (s, o) =>
            val ret: (String, JsValueWrapper) = s.toString -> JsBoolean(o)
            ret
        }.toSeq:_*)
}

implicit val mapFormat: Format[Map[Int, Boolean]] = Format(mapReads, mapWrites)

I have tested it with play 2.3. I'm not sure if it's the best approach to have a Map[Int, Boolean] on server side and a json object with string -> boolean mapping on the client side, though.