Jerkson. Serializing map to JsValue - scala

Question

I am using built-in jerson with playframework 2, and all I want is to serialize map, containing values of different type:
object AppWriter extends Writes[Application] {
def writes(app: Application): JsValue = {
Json.toJson(Map(
"id" -> app.getId.toString,
"name" -> app.getName,
"users" -> Seq(1, 2, 3)
))
}
}
In this case I have:
No Json deserializer found for type scala.collection.immutable.Map[java.lang.String,java.lang.Object].
Try to implement an implicit Writes or Format for this type
Navigatin through framework code shows that there is serializer for Map[String,V] types implicit def mapWrites[V] .. , but I cannot understand why it doesn't applied.
Can anybody help me?
UPD: I found simple workaround:
object AppWriter extends Writes[Application] {
def writes(app: Application): JsValue = {
Json.toJson(Map[String, JsValue](
"id" -> JsNumber(BigDecimal(app.getId)),
"name" -> JsString(app.getName),
"users" -> JsArray(Seq(1, 2, 3).map(x => JsNumber(x)))
))
}
}
but this is not so elegant...

Answer 1

The standard way to do this is by creating a JsObject from the individual key-value pairs for the fields—not by putting the pairs into a map. For example, assuming your Application looks like this:
case class Application(getId: Int, getName: String)
You could write:
import play.api.libs.json._, Json.toJson
implicit object AppWriter extends Writes[Application] {
def writes(app: Application): JsValue = JsObject(
Seq(
"id" -> JsNumber(app.getId),
"name" -> JsString(app.getName),
"users" -> toJson(Seq(1, 2, 3))
)
)
}
And then:
scala> toJson(Application(1, "test"))
res1: play.api.libs.json.JsValue = {"id":1,"name":"test","users":[1,2,3]}
Note that you don't need to spell out how to serialize the Seq[Int]—the default Format instances will do the work for you.