$mapps = Student_mapping::select('student_id');

    if($request->session_id) {
        $mapps = $mapps->where('session', '=', $request->session_id);
    }
    if($request->class_id) {
        $mapps = $mapps->where('class_id', '=', $request->class_id);

        if($request->group_id) {
            $mapps = $mapps->where('group_id', '=', $request->group_id);

            if($request->section_id) {
                $mapps = $mapps->where('section_id', '=', $request->section_id);
            }
        }
    }

    $mapps = $mapps->get();

    $students = [];

    foreach($mapps as $map) {
        if($map->student)
        {
            $students[] = Student::find($map->student_id);
        }
    }