gistfile1.scala

// class definitions

trait SubUnit
case class Dept(name: String, manager: Employee, subUnits: List[SubUnit]) extends SubUnit
case class Employee(name: String, salary: Double) extends SubUnit

// typeclass instance

object SubUnitProtocol extends DefaultProtocol {
  import JsonSerialization._
  implicit object SubUnitFormat extends Format[SubUnit] {
    def reads(json: JsValue): SubUnit = json match {
      case j@JsObject(m) => m.keys.size match {
        case 2 => fromjson[Employee](j)
        case _ => fromjson[Dept](j)
      }

      case _ => throw new RuntimeException("JsObject expected")
    }

    def writes(s: SubUnit): JsValue = s match {
      case d: Dept => tojson(d)
      case e: Employee => tojson(e)
    }
  }
}

import SubUnitProtocol._
implicit val DeptFormat: Format[Dept] = 
  lazyFormat(asProduct3("name", "manager", "subUnits")(Dept)(Dept.unapply(_).get))

implicit val EmployeeFormat: Format[Employee] = 
  asProduct2("name", "salary")(Employee)(Employee.unapply(_).get)

// test script

describe("Serialization of list of traits") {
  val e1 = Employee("Mary G.", 1000)
  val e2 = Employee("John P.", 5000)
  val e3 = Employee("Peter P.", 5000)

  val d1 = Dept("Finance", e1, List(e1, e2))
  val d2 = Dept("Accounting", e2, List(e1, e2, e3))
  val d3 = Dept("Systems", e2, List(d1, d2))

  it ("should serialize") {
    fromjson[Dept](tojson(d1)) should equal(d1)
    fromjson[Dept](tojson(d3)) should equal(d3)
  }
}