public function mapSkuToRowId($sku)
    {

        // query weather or not the SKU has been mapped
        if (isset($this->skuRowIdMapping[$sku])) {
            return $this->skuRowIdMapping[$sku];
        }

        // throw an exception if the SKU has not been mapped yet
        throw new \Exception(sprintf('Found not mapped SKU %s', $sku));
    }