How to represent optional fields in spray-json?

Question

I have an optional field on my requests:

case class SearchRequest(url: String, nextAt: Option[Date])

My protocol is:

object SearchRequestJsonProtocol extends DefaultJsonProtocol {
    implicit val searchRequestFormat = jsonFormat(SearchRequest, "url", "nextAt")
}

How do I mark the nextAt field optional, such that the following JSON objects will be correctly read and accepted:

{"url":"..."}
{"url":"...", "nextAt":null}
{"url":"...", "nextAt":"2012-05-30T15:23Z"}

I actually don't really care about the null case, but if you have details, it would be nice. I'm using spray-json, and was under the impression that using an Option would skip the field if it was absent on the original JSON object.

Answer 1

You might have to create an explicit format (warning: psuedocodish):

object SearchRequestJsonProtocol extends DefaultJsonProtocol {
    implicit object SearchRequestJsonFormat extends JsonFormat[SearchRequest] {
        def read(value: JsValue) = value match {
            case JsObject(List(
                    JsField("url", JsString(url)),
                    JsField("nextAt", JsString(nextAt)))) =>
                SearchRequest(url, Some(new Instant(nextAt)))

            case JsObject(List(JsField("url", JsString(url)))) =>
                SearchRequest(url, None)

            case _ =>
                throw new DeserializationException("SearchRequest expected")
        }

        def write(obj: SearchRequest) = obj.nextAt match {
            case Some(nextAt) => 
                JsObject(JsField("url", JsString(obj.url)),
                         JsField("nextAt", JsString(nextAt.toString)))
            case None => JsObject(JsField("url", JsString(obj.url)))
        }
    }
}