Play Framework: No implicit format for Map

Question

Using Play 2.5 I cannot seem to serialize a Map[SomeCaseClass, String]

case class SomeCaseClass(value: String)

implicit val formatSomeCaseClass = Json.format[SomeCaseClass]

Json.toJson(Map[SomeCaseClass, String](SomeCaseClass("") -> ""))

Errors with

No Json serializer found for type scala.collection.immutable.Map[SomeCaseClass,String]. Try to implement an implicit Writes or Format for this type.

Unless i'm missing something obvious, there is an implicit format for that type immediately above.

If I try something more simple like:

Json.toJson(Something(""))
Json.toJson(Map[String, String]("" -> ""))

It works fine. What am I missing when using a Map with a more complex type e.g. SomeCaseClass?

Answer 1

I think the problem here comes from json. Maps get converted to JSON objects which consist of key/value pairs. The keys in those objects must be strings.

So a Map[String, T] can get converted to a json object but not an arbitrary Map[U, T].

Answer 2

@Jack Bourne is correct. A map is treated as a JsObject in play json and because of that the key must be serialisable to a string value.

Here is a sample code that you can use to define a map format

import play.api.libs.json._

case class SomeCaseClass(value: String)

implicit val formatSomeCaseClass = Json.format[SomeCaseClass]

Json.toJson(Map[SomeCaseClass, String](SomeCaseClass("") -> ""))

implicit val someCaseClassToStringFormat = new Format[Map[SomeCaseClass, String]] {
override def writes(o: Map[SomeCaseClass, String]): JsValue = {
  val tuples = o.toSeq.map {
    case (key, value) => key.value -> Json.toJsFieldJsValueWrapper(value)
  }
  Json.obj(tuples: _*)
}

override def reads(json: JsValue): JsResult[Map[SomeCaseClass, String]] = {
  val resultMap: Map[SomeCaseClass, String] = json.as[JsObject].value.toSeq.map {
    case (key, value) => Json.fromJson[SomeCaseClass](value) match {
      case JsSuccess(someCaseClass, _) => someCaseClass -> key
      case JsError(errors) => throw new Exception(s"Unable to parse json :: $value as SomeCaseClass because of ${JsError.apply(errors)}")
    }
  }(collection.breakOut)
  JsSuccess(resultMap)
 }
}