public function setRefreshToken($refresh_token, $client_id, $user_id, $expires, $scope = null)
    {
        // Get Client Entity
        $client = $this->em->getRepository('OAuth2ServerBundle:Client')->find($client_id);
        if (!$client) {
            return null;
        }

        // Create Refresh Token
        $refreshToken = new \OAuth2\ServerBundle\Entity\RefreshToken();
        $refreshToken->setToken($refresh_token);
        $refreshToken->setClient($client);
        $refreshToken->setUserId($user_id);
        $refreshToken->setExpires($expires);
        $refreshToken->setScope($scope);

        // Store Refresh Token
        $this->em->persist($refreshToken);
        $this->em->flush();
    }