Group by in laravel withcount orm

index.html

<?php

//in Model.php
public function contributes()
{
  return $this->hasmany('\App\ExamUserSelect', 'exam_id');
}

/* in controller
 * grouped by group_id
 *
*/

return Model::withcount(['contributes' => function($q){
  $q->select( \DB::raw('count(distinct(group_id))') );
}];