public function postEdit(Request $request, $id)
 {
     $qcm = Qcm::where('id', $id)->with('user', 'questions')->first();
     if (Auth::id() === $qcm->user->id) {
         $this->validate($request, ['name' => 'required|string', 'description' => 'required|string', 'subject_id' => 'required|in:' . implode(',', array_keys(Subject::toList())), 'questions' => 'required|array', 'valids_answers' => 'required|array|size_array:questions|answer_exists:answers', 'answers' => 'required|array']);
         $ret = DB::transaction(function () use($request, $qcm) {
             $datas = $request->all();
             $questions = $qcm->questions;
             $qcm->name = $datas['name'];
             $qcm->description = $datas['description'];
             $qcm->subject_id = $datas['subject_id'];
             $qcm->update();
             foreach ($questions as $q => $question) {
                 $question->question = $datas['questions'][$q];
                 $question->update();
                 foreach ($question->answers as $a => $answer) {
                     $answer->answer = $datas['answers'][$q][$a];
                     $answer->isValid = $datas['valids_answers'][$q] == $a;
                     $answer->update();
                 }
             }
             return true;
         });
         if ($ret) {
             Session::push('messages', 'success|Le QCM a bien été modifié');
         } else {
             Session::push('messages', "danger|Le QCM n'a pas été modifié");
         }
         return redirect(route('qcm::mine'));
     } else {
         Session::push('messages', 'danger|Vous ne pouvez pas modifier le QCM d\'un autre professeur');
         Auth::logout();
         return redirect(route('index'));
     }
 }