replace _id:BsonObjectId by id:String in play reactive mongo

gistfile1.scala

import play.api.libs.json._

case class User (id:String, name:String)

object JsonFormats {

  def mongoReads[T](r: Reads[T]) = {
    __.json.update((__ \ 'id).json.copyFrom((__ \ '_id \ '$oid).json.pick[JsString] )) andThen r
  }

  def mongoWrites[T](w : Writes[T]) = {
    w.transform( js => js.as[JsObject] - "id"  ++ Json.obj("_id" -> Json.obj("$oid" -> js \ "id")) )
  }

  implicit val userRead: Reads[User] = mongoReads[User](Json.reads[User])
  implicit val userWrites: Writes[User] = mongoWrites[User](Json.writes[User])
}