Play json looking not working well in this case - scala

Question

I have following code to work with play json
import play.api.libs.json.Json
trait A {
def x: Option[Int]
def y: Option[String]
}
case class A1(x: Option[Int] = Some(1), y: Option[String]) extends A
object PlayJsonBug {
def main(args: Array[String]): Unit = {
implicit val A1Reader = Json.reads[A1]
implicit val A1Writer = Json.writes[A1]
val str = """{"y":"xyz"}"""
val a: A1 = Json.fromJson[A1](Json.parse(str)).get
println(a)
}
}
case class A1 has default value for x which is Some (1).
when i parse {"y":"xyz"} ,the result is A1(None,Some(xyz)),I thought that the default value should be used here,that tis,the result should be A1(Some(1),Some(xyz))
Is this a bug?Or how I could work around this problem

Answer 1

Json.reads is a scala macro whose implementation doesn't care for default values since it only iterates through all of the class' case accessors. You will need to implement your own play.api.libs.json.Reads to support falling back to default parameters in case of not finding the value in json.
It could look something like this (for Play 2.5.x):
implicit object A1Reads extends Reads[A1] {
private val generatedReads = Json.reads[A1]
override def reads(json: JsValue): JsResult[A1] = {
// re-use reads, but replace None for x with default param
generatedReads.reads(json).map {
case A1(None, y) => A1(Some(1), y)
case valid => valid
}
}
}