完善我的雄辩查询（Laravel）

Edited for Clarity:

I need some way for this Query to only return distinct course_ids in the

->wherein('course_id', $array)

part of the query. It gets the total number of completed courses, but if a user has completed a course more than once it counts it toward the total. So if you want to know how many students have done a course, the number will be off if a student has completed a course more than once.

    public function report_count(){
    $array = \Session::get('course_report')['course'];
    return $this->hasOne('Tracking', 'roster_id')
        ->selectRaw('roster_id, count(*) as aggregate')
        ->where('status', 1)
        ->wherein('course_id', $array)
        ->groupBy('roster_id');

I've tried adding groupBy('course_id') at the end but it does not work.

Fixed. Changed the query to

return $this->hasOne('Tracking', 'roster_id')
        ->selectRaw('roster_id, count(distinct course_id) as aggregate')
        ->where('status', 1)
        ->wherein('course_id', $array)
        ->groupBy('roster_id');

per @Robin R suggestion, I went on a search of ways to utilize SQL in my selectRaw query instead of trying this the eloquent way. Thanks to all who helped me brainstorm though this one.