Writing JSON depending on instance type

Question

I have the following class structure in Play for Scala:

sealed trait Father
final case class Child1 (name: String, descrip: String) extends Father
final case class Child2 (age: Int, price: Int) extends Father

case class MyClass (someValue: Int, father: Father)

Now, I need to define the implicit JSON functions for MyClass:

  implicit val myClassWrite : Writes[MyClass] = (
     (JsPath \ "some").write[Int] and
     (JsPath \ "father").write[Father]
  ) (unlift(MyClass.unapply))

What I get is:

No Json serializer found for type Father. Try to implement an implicit Writes or Format for this type.

The thing is that there's no way to write a Writes JSON function for Father as it's a trait with no attributes. I can only write JSON functions for Child1 and Child2, but I still get the error. My intent is for Play to insert the JSON of Child1 or Child2 depending on the instance type. Is this something Play can do?

Play JSON can generate writer for the sealed trait directly. — cchantep

余杰水余杰水 · Accepted Answer · 2018-02-27T03:38:28

implicit val c1 = Json.format[Child1]
implicit val c2 = Json.format[Child2]

implicit val fatherWrites = new Writes[Father] {
  override def writes(o: Father) = o match {
    case x: Child1 => c1.writes(x)
    case x: Child2 => c2.writes(x)
  }
}

// test

val father : Father = Child1("aa","aaa")
Json.toJson(father).toString() // {"name":"aa","descrip":"aaa"}

Writing JSON depending on instance type

1 Answers