title: Laravel Validation : How to validate two related foreign key correspond or not
body: <p>I have student, grade and section table. Section belongs to grade. Student belongs to grade and section. Now when student has to be created, I get grade id and section id. I want to validate the section id if the section id belongs to same grade id as in request.</p>
  
  <p>grade table : id, title.
  section table : id, grade_id, title.
  student table : id, name, grade_id, section_id, ....</p>
  
  <p>Answer: 'exists:sections,id,grade_id,' . $this->input("grade_id")</p>