Serialize objects as JSON primitive type in Scala (Play Framework)

The documentation indicates how to provide custom Writes, there to MyTrait to serialize only it's inner value.

sealed trait MyTrait {
  def value: Int
}

case object A extends MyTrait {
  val value = 0
}

case object B extends MyTrait {
  val value = 1
}

case object C extends MyTrait {
  val value = 2
}

object MyTrait {
  import play.api.libs.json._

  def apply(value: Int): MyTrait = value match {
    case 0 => A
    case 1 => B
    case 2 => C
    case _ => C
  }

  def unapply(arg: MyTrait): Int = arg.value

  // ---v
  implicit val writes: Writes[MyTrait] =
    Writes[MyTrait] { v => JsNumber(v.value) }
}

Then when serializing MyTrait instances (note the type ascription bellow is required as Writes is invariant):

Json.toJson(A: MyTrait)
// --> res2: play.api.libs.json.JsValue = 0

And so about Test class:

case class Test(value: MyTrait)

object Test {
  import play.api.libs.json._

  implicit val writes: OWrites[Test] = Json.writes[Test]
}

Json.toJson(Test(A))
// ---> res1: play.api.libs.json.JsValue = {"value":0}

I would rather recommend to have a look at Enumeratum for the enumerated type MyTrait, or to refactor MyTrait as a Value class and use Json.valueFormat as below.

import play.api.libs.json._

final class MyTrait private(val value: Int) extends AnyVal

object MyTrait {
  val A = new MyTrait(0)
  val B = new MyTrait(1)
  val C = new MyTrait(2)

  implicit val format: Format[MyTrait] = Json.valueFormat[MyTrait]
}

case class Test(value: MyTrait)

object Test {
  implicit val format: OFormat[Test] = Json.format
}

scala> Json.toJson(Test(MyTrait.A))
res0: play.api.libs.json.JsValue = {"value":0}