public function id($disallowNull = false)
        {
            $id = $this->get($this->_getIdColumnName());

            if ($disallowNull) {
                if (Arrays::is($id)) {
                    foreach ($id as $id_part) {
                        if ($id_part === null) {
                            throw new Exception('Primary key ID contains null value(s)');
                        }
                    }
                } else if ($id === null) {
                    throw new Exception('Primary key ID missing from row or is null');
                }
            }

            return $id;
        }