flattenJsonTree.scala

  def flattenObject(obj: JsObject, prefix: Option[String] = None) :Seq[(String, JsValue)] = {
    obj.fields.toList match {
      case ("$date",_) :: Nil if prefix.isDefined => Seq(prefix.get -> obj)
      case ("$date",_) :: Nil => throw new Exception("Can't save $date at the first level.")
      case fs => fs.flatMap { t =>
        val name = prefix.map(_ + ".").getOrElse("") + t._1
        t._2 match {
          case obj: JsObject => flattenObject(obj, Some(name))
          case value => Seq(name -> value)
        }
      }
    }
  }