public function getArticlesByTagDesc(string $tag, int $size = null) : Collection
    {
        $size = $size ?? $this->articles->count();
        $articles = $this->articles;
        return $this->tag_list->get($tag)->reverse()->map(function ($item) use ($articles) {
            return $articles->get($item);
        })->values()->take($size);
    }