How to represent optional fields in spray-json?

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.

James Marble

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)))
        }
    }
}

Works for me (spray-json 1.1.1 scala 2.9.1 build)

import cc.spray.json._
import cc.spray.json.DefaultJsonProtocol._

// string instead of date for simplicity
case class SearchRequest(url: String, nextAt: Option[String])

// btw, you could use jsonFormat2 method here
implicit val searchRequestFormat = jsonFormat(SearchRequest, "url", "nextAt")

assert {
  List(
    """{"url":"..."}""",
    """{"url":"...", "nextAt":null}""",
    """{"url":"...", "nextAt":"2012-05-30T15:23Z"}""")
  .map(_.asJson.convertTo[SearchRequest]) == List(
    SearchRequest("...", None),
    SearchRequest("...", None),
    SearchRequest("...", Some("2012-05-30T15:23Z")))
}

Use NullOptions trait to disable skipping nulls: https://github.com/spray/spray-json#nulloptions Example: https://github.com/spray/spray-json/blob/master/src/test/scala/spray/json/ProductFormatsSpec.scala

Don't know if this will help you but you can give that field a default value in the case class definition, so if the field is not in the json, it will assign the default value to it.

Easy.

import cc.spray.json._
trait MyJsonProtocol extends DefaultJsonProtocol {
  implicit val searchFormat = new JsonWriter[SearchRequest] {
    def write(r: SearchRequest): JsValue = {
      JsObject(
        "url" -> JsString(r.url),
        "next_at" -> r.nextAt.toJson,
      )
    }
  }
}

class JsonTest extends FunSuite with MyJsonProtocol {
test("JSON") {
    val search = new SearchRequest("www.site.ru", None)
    val marshalled = search.toJson
    println(marshalled)
  }
}

For anyone who is chancing upon this post and wants an update to François Beausoleil's answer for newer versions of Spray (circa 2015+?), JsField is deprecated as a public member of JsValue; you should simply supply a list of tuples instead of JsFields. Their answer is spot-on, though.

来源：https://stackoverflow.com/questions/10819344/how-to-represent-optional-fields-in-spray-json

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