Play Json Reads nested generic serialized Json - json

Question

Consider the following JSON
{
"a": "{\"b\": 12, \"c\": \"test\"}"
}
I would like to define a generic reads Reads[Outer[T]] for this kind of serialized Json
import play.api.libs.json.{Json, Reads, __}
final case class Outer[T](inner: T)
final case class SpecializedInner(b: Int, c: String)
object SpecializedInner {
implicit val reads: Reads[SpecializedInner] = Json.reads[SpecializedInner]
}
object Outer {
implicit def reads[T](implicit readsT: Reads[T]): Reads[Outer[T]] = ???
}
How can I achieve my goal? I tried to flatMap a string-reads for the field "outer.a" but got stuck since I am not able to produce a Reads[T] from the validated JSON
object Outer {
implicit def reads[T](implicit readsT: Reads[T]): Reads[Outer[T]] =
(__ \ "a").read[String].flatMap(x => Json.parse(x).validate[T])
}

Answer 1

You need just add map inside Outer.reads construction after validate[T] invocation.
Please, see next code for example:
object App {
final case class Outer[T](inner: T)
object Outer {
implicit def reads[T](implicit innerReads: Reads[T]): Reads[Outer[T]] = { json: JsValue =>
json.validate[String].flatMap(string => Json.parse(string).validate[T].map(Outer.apply[T]))
}
}
final case class Root[T](a: Outer[T])
object Root {
implicit def reads[T](implicit innerReads: Reads[T]): Reads[Root[T]] = Json.reads
}
final case class SpecializedInner(b: Int, c: String)
object SpecializedInner {
implicit val reads: Reads[SpecializedInner] = Json.reads
}
def main(args: Array[String]): Unit = {
val rawJson = "{\"a\": \"{\\\"b\\\": 12, \\\"c\\\": \\\"test\\\"}\"}"
println(Json.parse(rawJson).validate[Root[SpecializedInner]])
}
}
Which produced next result in my case:
JsSuccess(Root(Outer(SpecializedInner(12,test))),)
Hope this helps!