public function GetCustomerNumber()
    {
        $sCustNr = parent::GetCustomerNumber();
        if (empty($sCustNr)) {
            $oShop = TdbShop::GetInstance();
            $sCustNr = $oShop->GetNextFreeCustomerNumber();
            $aData = $this->sqlData;
            $aData['customer_number'] = $sCustNr;
            $this->LoadFromRow($aData);
        }

        return $this->fieldCustomerNumber;
    }