Группировка значений при join

Всем добрый вечер. Мне нужно сгрупировать значения при join для того чтобы ключ коллекции был 'key' Мой код:

    $result = $product->with(['filters' => function ($q) {
        $q->join('specifications', 'filters.specification_id', '=', 'specifications.id')
            ->select('specifications.slug as key', 'filters.filter_min_value as value', 'filters.filter_max_value as max_value', 'filtereds.postfix');
    }]);

Я получаю такую коллекцию

"filters" => Illuminate\Database\Eloquent\Collection {#2235 ▼
      #items: array:53 [▼
        0 => App\Filter {#2548 ▶}
        1 => App\Filter {#2549 ▶}
        2 => App\Filter {#2550 ▶}
        3 => App\Filter {#2551 ▶}
        4 => App\Filter {#2552 ▶}
        5 => App\Filter {#2553 ▶}
        6 => App\Filter {#2554 ▶}
        7 => App\Filter {#2555 ▶}
        8 => App\Filter {#2556 ▶}
        9 => App\Filter {#2557 ▶}
        10 => App\Filter {#2558 ▶}

Хотелось бы увидеть

    "filters" => Illuminate\Database\Eloquent\Collection {#2235 ▼
#items: array:53 [▼
    'key' => App\Filter {#2548 ▶}
    'key' => App\Filter {#2548 ▶}
    'key' => App\Filter {#2548 ▶}
    'key' => App\Filter {#2548 ▶}
    'key' => App\Filter {#2548 ▶}

Как можно достич данного результата на laravel?