trait ISearchKey {
    var id:String = ""
}

val fld = new CrystalFieldInfo("Field1") with ISearchKey
  fld.id = "Id1"
  implicit val formats = Serialization.formats(NoTypeHints)
  val ser = write[CrystalFieldInfo with ISearchKey](fld)
  println(ser)

implicit val formats = DefaultFormats + FieldSerializer[ISearchKey]
val ser = write[CrystalFieldInfo with ISearchKey]
println(ser) // should include the "id" field from the ISearchKey trait