Serialize extended class to JSON in Scala with Play Framework serializer - json

Question

I would like to serialize an extended class in scala and
i have some test code..
import org.specs2.mutable._
import org.specs2.runner._
import org.junit.runner._
import play.api.libs.json.Json
#RunWith(classOf[JUnitRunner])
class JsonSerializerTest extends Specification {
class A(val s1: String)
case class B(s2: String) extends A("a")
"Application" should {
"serialize class to JSON" in {
implicit val bWrites = Json.writes[B]
implicit val bReads = Json.reads[B]
val bClass = B("b")
println(bClass.s1 + " " + bClass.s2)
val serialized = Json.toJson[B](bClass)
val s1 = (serialized \ "s1").asOpt[String]
s1 should beSome[String]
}
}
}
In this case test print:
a b
Application should
'None' is not Some
java.lang.Exception: 'None' is not Some
It means that s1 field from parent class were not serialized.
The solution
class A(val s1: String)
case class B(override val s1: String, s2: String) extends A(s1)
mostly unacceptable because in real application classes have a lot of fields and specifying them explicitly every time when i extend class complicates the code.
Is there any other solution for this case?

Answer 1

you can manually create the json serializers (described here: https://www.playframework.com/documentation/2.5.x/ScalaJsonCombinators)
the problem with your version is that Json.writes and Json.reads are macros that are looking specifically at the constructor of the case class, and building the serializer from that (so superclass arguments aren't captured). you could copy and roll your own version of the macro: https://github.com/playframework/playframework/blob/d6c2673d91d85fd37de424951ee5ad9f4f4cce98/framework/src/play-json/src/main/scala/play/api/libs/json/JsMacroImpl.scala
lastly, you can make a function that takes the result of Json.writes and Json.reads, and adds on the shared fields you want. something like:
object A {
def writesSubclass[T](writer: Writes[T]): Writes[T] = new Writes[T] {
def writes(t: T) = Json.obj("s1" -> t.s1) ++ writer.writes(t).as[JsObject]
}
}
implicit val bWrites = A.writesSubclass(Json.writes[B])
depending on how often your A gets extended, this might be your best bet.