public function index(MasterSupervisorRepository $masters,
                          SupervisorRepository $supervisors)
    {
        $masters = collect($masters->all())->keyBy('name')->sortBy('name');

        $supervisors = collect($supervisors->all())->sortBy('name')->groupBy('master');

        return $masters->each(function ($master, $name) use ($supervisors) {
            $master->supervisors = $supervisors->get($name);
        });
    }