function getAuthors() {
        $sql = 'SELECT *
                FROM author';
        return $this->db->selectAll($sql);
    }

    function getCategories() {
        $sql = 'SELECT *
                FROM category';
        /*$sql = 'SELECT student.*, class.name AS class '
                . 'FROM student '
                . 'INNER JOIN class '
                . 'ON student.class_id = class.id';*/
        return $this->db->selectAll($sql);
    }