$selector = ORM::factory('part')->where('category_id', '=', $category)
            ->join('parts_products')->on('parts.id', '=', 'parts_products.part_id')
            ->group_by('parts.id')
            ->where('parts_products.in_stock', '=', '1');
        var_dump($selector->reset(false)->count_all(), count($selector->find_all()));die;

/* return values */

string(1) "1" int(88)