public function getTransport() {
        if ($this->transport === null || !$this->transport instanceof xPDOTransport) {
            $transportClass= $this->xpdo->getOption('xPDOTransport.class', null, 'xPDOTransport');
            $this->transport= new $transportClass($this);
            if ($this->transport === null || !$this->transport instanceof xPDOTransport) {
                $this->xpdo->log(xPDO::LOG_LEVEL_ERROR, "Could not load xPDOTransport [{$transportClass}] class.");
            }
        }
        return $this->transport;
    }