demo.scala

package demo

import java.time.Instant
import java.util.UUID

import demo.RootJsonFormatWithDefault.toRootJsonFormatWithDefault
import spray.json._

final case class User(id: String, name: String, timestamp: Long)

trait JsonProtocol extends DefaultJsonProtocol {
  // use default json format for user and extend this with two default values
  implicit val userFormat = jsonFormat3(User)
    .withDefault("_id", "some-random-id")
    .withDefault("timestamp", System.currentTimeMillis)
}

RootJsonFormatWithDefault.scala

package demo

import spray.json._

import scala.language.implicitConversions

class RootJsonFormatWithDefault[T](inner: RootJsonFormat[T]) {
  def withDefault[V](key: String, defaultValue: => V)(implicit writer: JsonWriter[V]): RootJsonFormat[T] = new RootJsonFormat[T] {
    override def read(json: JsValue) = {
      if (json.asJsObject.fields.contains(key)) inner.read(json)
      else inner.read(JsObject(json.asJsObject.fields + (key -> writer.write(defaultValue))))
    }

    override def write(obj: T) = inner.write(obj)
  }
}

object RootJsonFormatWithDefault {
  implicit def toRootJsonFormatWithDefault[T](inner: RootJsonFormat[T]): RootJsonFormatWithDefault[T]
    = new RootJsonFormatWithDefault(inner)
}