public function generatePK()
    {
        $format = $this->pkFormat;
        $pk     = $this->_properties['primary_key'];
        if (isset($pk[1]))
            throw new Exception(sprintf(self::E_Composit, get_called_class(), __FUNCTION__), 1);

        $pk     = array_shift($pk);
        if (!$pk)
            throw new Exception(sprintf(self::E_PrimaryKey, get_called_class()), 1);

        $last = (int) $this
            ->select(($format['serial']?
                'right('.$pk.', '.$format['serial'].')':
                $pk).' as last')
            ->order($pk.' desc')
            ->read(1)
            ->last;

        // compile prefix format
        if (preg_match('/\{(?<date>.+)\}/', $format['prefix'], $match))
            $newPK = preg_replace('/\{.+\}/', date($match['date']), $format['prefix']);
        else
            $newPK = $format['prefix'];

        $newPK .= trim(str_pad($last+1, $format['serial'], $format['prefix']?'0':' ', STR_PAD_LEFT));

        return $newPK;
    }