protected function initializeProductWebsite(array $attr)
    {

        // try to load the product website relation with the passed product/website ID
        if ($this->loadProductWebsite($attr[MemberNames::PRODUCT_ID], $attr[MemberNames::WEBSITE_ID])) {
            // throw a runtime exception, if the relation already exists
            throw new \RuntimeException(
                sprintf(
                    'Product website relation %d => %d already exsits',
                    $attr[MemberNames::PRODUCT_ID],
                    $attr[MemberNames::WEBSITE_ID]
                )
            );
        }

        // otherwise simply return the attributes
        return $attr;
    }