Play JSON Serialization and Trait Methods

example.scala

// Trait that adds the 'computeSum' method 
trait HasSum {
  
  val a: Int
  
  val b: Int
  
  def computeSum() = a + b
  
}

// Generic serialization for classes inheriting the HasSum trait
trait HasSumWrites {

  implicit def hasSumWrites[T <: HasSum](implicit w: Writes[T]): Writes[T] = (
    (JsPath).write[T] and
    (JsPath \ "sum").write[Int]
  )(t => (t, t.computeSum()))

}

// Two example case classes extending HasSum
case class Foo(propertyA: String, a: Int, b: Int) extends HasSum
case class Bar(propertyB: String, a: Int, b: Int) extends HasSum

// Their companions, explicitely defining a serialization based on that of HasSumWrites
object Foo extends HasSumWrites {
  implicit val fooWrites = hasSumWrites(Json.writes[Foo])
}

object Bar extends HasSumWrites {
  implicit val barWrites = hasSumWrites(Json.writes[Bar])
}

// Example
val foo = Foo("some value", 1, 2)
val bar = Bar("some other value", 4, 5)
println(Json.toJson[Foo](foo).toString) 
println(Json.toJson[Bar](bar).toString)

// Returns
// {"propertyA":"some value","a":1,"b":2,"sum":3}
// {"propertyB":"some other value","a":4,"b":5,"sum":9}