public function getUrl(ExtReference $extReference)
    {
        if (!isset($this->mapping[$extReference->getRef()])) {
            throw new \InvalidArgumentException(
                sprintf(
                    'Could not create URL from extref "%s"',
                    json_encode($extReference)
                )
            );
        }

        return $this->router->generate(
            $this->mapping[$extReference->getRef()].'.get',
            ['id' => $extReference->getId()],
            UrlGeneratorInterface::ABSOLUTE_URL
        );
    }