public function listByCategory($cat = null)
    {
        $result = $this->db->query('SELECT id, title, resume, category, author, link  
                                      FROM '.$this->articleTable.' 
                                      WHERE category = :cat', [':cat'=>$cat]);
        if (isset($result[0])) {
            return $this->comList($result);
        }
        return false;
    }