Adding a JSON field to a Scala case class

Question

I have a Scala case class

case class NumericParam(minValue: Option[Int] = None,
                        maxValue: Option[Int] = None,
                        decimalPlaces: Option[Int] = None,
                        signed: Option[Boolean] = None,
                        hintText: Option[String] = None)

and its companion object, where I defined an implicit writes method

object NumericParam {
    implicit val writes = new Writes[NumericParam] {
        override def writes(value: NumericParam): JsValue = {
          Json.obj(
            "dataType" -> "Numeric",
            "minValue" -> value.maxValue,
            "maxValue" -> value.maxValue,
            "decimalPlaces" -> value.decimalPlaces,
            "signed" -> value.signed,
            "hintText" -> value.hintText
          )
        }
    }
}

I am adding the field dataType. Is there any way to use the macro-derived Writes value (from Json.writes[NumericParam]) and just add the additional dataType field?

Michael Zajac · Accepted Answer · 2016-02-11 01:10:57Z

You can use Writes#transform to do it. One way would be to use a function JsValue => JsValue. The safe way:

implicit val writes = Json.writes[NumericParam] transform { js =>
  js match {
    case obj: JsObject => obj + ("dataType" -> JsString("Numeric"))
    case _ => js
  }
}

However, we really know that js should always be a JsObject, since we're operating on a specific type, so we can shorten it.

implicit val writes = Json.writes[NumericParam]
   .transform(_.as[JsObject] + ("dataType" -> JsString("Numeric")))

Example:

scala> val num = NumericParam(Some(1), Some(10), Some(2), Some(false), Some("test"))
num: NumericParam = NumericParam(Some(1),Some(10),Some(2),Some(false),Some(test))

scala> Json.toJson(num)
res5: play.api.libs.json.JsValue = {"minValue":1,"maxValue":10,"decimalPlaces":2,"signed":false,"hintText":"test","dataType":"Numeric"}

To make the above more type-safe and generic, we can use some implicit magic to extend OWrites (which always writes to JsObject).

implicit class OWritesExt[A](owrites: OWrites[A]) {

  /** Add a (key, value) pair to the JSON object,
   *  where the value is constant.
   */
  def withConstant[B : Writes](key: String, value: B): OWrites[A] = 
    withValue(key, _ => value)

  /** Add a key, value pair to the JSON object that is written, where
   *  the value depends on the original object.
   */
  def withValue[B : Writes](key: String, value: A => B): OWrites[A] = 
    new OWrites[A] {
      def writes(a: A): JsObject = owrites.writes(a) ++ Json.obj(key -> value(a))
    }

}

Usage:

implicit val writes = Json.writes[NumericParam]
    .withValue("type", _.getClass.toString)
    .withConstant("other", "something")

scala> Json.toJson(NumericParam(Some(1)))
res6: play.api.libs.json.JsValue = {"minValue":1,"type":"class NumericParam","other":"something"}

Now you can scrap some of the original boilerplate, and chain calls like this together. Now I'm just wondering why I haven't been doing this all along.

That is what I was looking for. I thought it would involve transform, but could not figure out how. Thanks.

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