title: Laravel validation not exists in table
body: <p>I have two tables, first one is students and second one is lesson_student table. They have m2m relationship between students and lessons tables. I want to check, if student_id exists in students table and not exists in lesson_student table, insert it. I use <code>exists</code> to check in students table but i have no idea how to check it is not exists in lesson_student table.</p>
  
  <pre><code>public function store(Request $request, Lesson $lesson) {
      $rules = [
          'student_id'    =&gt; 'required|exists:students,id'
      ];
  
      $this-&gt;validate($request, $rules);
  
      $lesson-&gt;students()-&gt;attach($request-&gt;student_id);
      return $this-&gt;showAll($lesson-&gt;students);
  }
  </code></pre>
  
  <p>btw i am using laravel 5.6</p>