protected function saveCover(ArticleRequest $request, $article)
 {
     if (!$request->hasfile('cover')) {
         return $article->cover;
     }
     $file = $request->file('cover');
     if (!$file->isValid()) {
         throw new Exception(trans('strings.image_not_invalid'));
     }
     $filename = $article->slug . '.' . $file->getClientOriginalExtension();
     $file->move(public_path('images/covers'), $filename);
     return url('images/covers/' . $filename);
 }