Play JSON with sealed trait / case classes: infinite recursion - scala

Question

I have a code where I try to customize JSON serialization of a bunch of case classes by defining a custom Writes for the base trait. I'm getting infinite recursion / stack overflow.
I created a simplified sample - if somebody knows how to fix it, please let me know.
import play.api.libs.json._
sealed trait Person {
val name: String
}
final case class Teacher(name: String, salary: Int) extends Person
final case class Student(name: String, grade: Int) extends Person
implicit val teacherWrites: Writes[Teacher] = Json.writes[Teacher]
implicit val studentWrites: Writes[Student] = Json.writes[Student]
val ThePersonWrites: Writes[Person] = Writes(person => {
Json.writes[Person].writes(person).as[JsObject] - "_type"
})
implicit val personWrites: Writes[Person] = ThePersonWrites
val people = List[Person] (
Teacher("Jane Doe", 40000),
Student("Alice", 5),
Student("Bob", 7)
)
Json.prettyPrint(Json.toJson(people))

Answer 1

You need play-json-derived-codecs
import play.api.libs.json._
import julienrf.json.derived
sealed trait Person {
val name: String
}
object Person {
implicit val jsonFormat: OFormat[Person] = derived.oformat[Person]()
}
final case class Teacher(name: String, salary: Int) extends Person
final case class Student(name: String, grade: Int) extends Person
val people = List[Person] (
Teacher("Jane Doe", 40000),
Student("Alice", 5),
Student("Bob", 7)
)
println(Json.prettyPrint(Json.toJson(people)))
See here the scalafiddle

Answer 2

This should do it:
import play.api.libs.json._
sealed trait Person {
val name: String
}
final case class Teacher(name: String, salary: Int) extends Person
final case class Student(name: String, grade: Int) extends Person
implicit val teacherWrites: Writes[Teacher] = Json.writes[Teacher]
implicit val studentWrites: Writes[Student] = Json.writes[Student]
implicit val personWrites: Writes[Person] = Writes[Person] {
case t: Teacher => Json.toJson(t)(teacherWrites)
case s: Student => Json.toJson(s)(studentWrites)
}
val people = List[Person] (
Teacher("Jane Doe", 40000),
Student("Alice", 5),
Student("Bob", 7)
)
Json.prettyPrint(Json.toJson(people))
The trick is adding teacherWrites and studentWrites explicitly. Because they are both Persons, before it was recognizing them as such and calling your personWrites again, hence the stack overflow.