gistfile1.scala

val getLanguageFile = (__ \ "codeUrl").json.copyFrom(
    (__ \ "files").json.pick[JsObject].map{ js => 
      js.fields.collectFirst{ case (k, v) if(v \ "language" != JsNull) => v \ "raw_url" }.get 
    }
  )
  
val getQuestionFile = (__ \ "questionUrl").json.copyFrom(
    (__ \ "files").json.pick[JsObject].map{ js => 
      js.fields.collectFirst{ case (k, v) if(v \ "language" == JsNull) => v \ "raw_url" }.get 
    }
  )
  
val fullTransf = (
  (__ \ "url").json.pickBranch and 
  getQuestionFile and 
  getLanguageFile
).reduce


tonJson.transform(fullTransf) => JsResult(json)