Scala/Play: modify Json.format macro behaviour for single field

Question

I have a big case class with 10+ fields, that represents a JSON payload, that comes from user. Most of fields are optional, so I'm using Option in this cases (Option[String] for string fields. And that is nice approach, until I need an optional sequence. I think that writing Option[Seq[String]] is weird, cause empty sequence is enough to show that there is no data (for any reason). I could handle it in manual way, though:

implicit val reads = new Reads[MyCaseClass] {
  def reads(js: JsValue): JsResult[MyCaseClass] = {
    JsSuccess(MyCaseClass(
      (js \ "unit_code").as[String],
      // other fields omited
      (js \ "positions").asOpt[Seq[String]] match {
        case Some(seq: Seq[String]) => seq
        case None => Seq.empty[String]
      }
    ))
  }
}

But I don't won't to write all this stuff manually. There could be mistakes, I need to test it separately, and, for sure, it takes much more time, rather than to write implicit val f = Json.format[MyCaseClass].

Is there any option to separately handle only one field, that other fields handling let on the default macro?

Answer 1

Thanks for user 'cchantep' to pointing to Json transformers. So that is how I solve the issue:

Case class:

case class MyCaseClass(unit_code: String, positions: Seq[String] = Seq.empty)

Companion object

object MyCaseClass {
  private val readsTransformer: Reads[JsObject] =  __.json.update(
    __.read[JsObject]
      .map{ o =>
        if (o.keys.exists(p => p.equals("positions"))) {
          o
        } else {
          o ++ Json.obj("positions" -> JsArray())
        }
      }
  )
  implicit val readsImplicit: Reads[MyCaseClass] = readsTransformer.andThen(Json.reads[MyCaseClass])
  implicit val writesImplicit: OWrites[MyCaseClass] = Json.writes[MyCaseClass]
}

That looks a bit cumbersome, but it is possible to write common function that creates transformer for exact field, so in each companion object it wouldn't be so verbose.