ScalaJsonTransformers.scala

object Main extends App {
  import play.api.libs.json._
  import play.api.libs.json.Reads._

  val json = Json.obj(
    "foo" -> "bar",
    "user" -> Json.obj(
      "name" -> "Mike",
      "age" -> 45,
      "food" -> "pizza"
    )
  )
  val userNameAndAge = (__ \ "user").json.update(
    __.read[JsObject].map { user =>
      val fields = user.fields.filter {
        case (field, _) => field == "name" || field == "age"
      } // Seq((name,"Mike"), (age,45))
      JsObject(fields)
    }
  )
  json.transform(userNameAndAge)
  // Now... This yields:
  // JsSuccess({"foo":"bar","user":{"name":"Mike","age":45,"food":"pizza"}},/user)
  // ... instead of (and what I'd like to have):
  // JsSuccess({"foo":"bar","user":{"name":"Mike","age":45}},/user)
  
  // Why?
}

proptip from @OlegYch: https://github.com/mandubian/play-json-zipper