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import ai.x.play.json.Jsonx // for >22 case class fields

val json = """{"item_id":126,"misc_id":668,"value": 10}"""

val formatter: Format[TestItem] = Jsonx.formatCaseClass[TestItem]

val reader = new Reads[TestItem] {
  override def reads(json: JsValue): JsResult[TestItem] = {
    json match {
      case o: JsObject =>
        val translated = o.fields
          .map({ case (k, v) => (StringUtil.camelcase(k), v) }) // snake to camel
          .map({ case (k, v) if k == "itemId" => ("id", v) case x => x }) // a custom mapping
        formatter.reads(JsObject(translated)) // call the original behavior with our key transformations
      case _ =>
        // will never happen
        formatter.reads(json)
    }
  }
}
Json.parse(json).as[TestItem](reader)