How to deserialize a Map of Map with Play - scala

Question

When I execute in the REPL, it works (due to implicit mapWrites in the scope):
scala> Map("a"->1l, "b"->2l)
res0: scala.collection.immutable.Map[String,Long] = Map(a -> 1, b -> 2)
scala> Map("c" -> res0, "d" -> res0)
res1: scala.collection.immutable.Map[String,scala.collection.immutable.Map[String,Long]] = Map(c -> Map(a -> 1, b -> 2), d -> Map(a -> 1, b -> 2))
scala> import play.api.libs.json._
import play.api.libs.json._
scala> Json.toJson(res1)
res2: play.api.libs.json.JsValue = {"c":{"a":1,"b":2},"d":{"a":1,"b":2}}
Why my code still doesn't compile (it's the same type as in the REPL) ?
No Json deserializer found for type Map[String,Map[String,Long]]. Try to implement an implicit Writes or Format for this type.
[edit] I've found a workaround but i don't understand why i need it :
implicit def mapWrites = Writes[Map[String,Map[String,Long]]] ( m => Writes.mapWrites(Writes.mapWrites[Long]).writes(m))

Answer 1

Play 2.1 JSON API does not provide a serializer for the Type Map[String, Object].
Define case class and Format for the specific type instead of Map[String, Object]:
// { "val1" : "xxx", "val2" : ["a", "b", "c"] }
case class Hoge(val1: String, val2: List[String])
implicit val hogeFormat = Json.format[Hoge]
If you don't want to create case class.
The following code provides JSON serializer/deserializer for Map[String, Object]:
implicit val objectMapFormat = new Format[Map[String, Object]] {
def writes(map: Map[String, Object]): JsValue =
Json.obj(
"val1" -> map("val1").asInstanceOf[String],
"val2" -> map("val2").asInstanceOf[List[String]]
)
def reads(jv: JsValue): JsResult[Map[String, Object]] =
JsSuccess(Map("val1" -> (jv \ "val1").as[String], "val2" -> (jv \ "val2").as[List[String]]))
}
Jackson doesn't know how to deserialize the Scala collection class scala.collection.immutable.Map, because it doesn't implement any of the Java collection interfaces.
You can either deserialize to a Java collection:
val mapData = mapper.readValue(jsonContent, classOf[java.util.Map[String,String]])
or add the Scala module to the mapper:
mapper.registerModule(DefaultScalaModule)
val mapData = mapper.readValue(jsonContent, classOf[Map[String,String]])
or you can also try like this
import play.api.libs.json._
val a1=Map("val1"->"a", "val2"->"b")
Json.toJSon(a1)
Because a1 is just Map[String,String] that works OK.
But if I have something more complex like where I have Map[String,Object], that doesn't work:
val a = Map("val1" -> "xxx", "val2"-> List("a", "b", "c"))
Json.toJSon(a1)
>>> error: No Json deserializer found for type scala.collection.immutable.Map[String,Object]
I found that I can do something like the following:
val a2 = Map("val1" -> Json.toJson("a"), "val2" -> Json.toJson(List("a", "b", "c")))
Json.toJson(a2)
And that works.`