public function setAccessToken($oauth_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 Access Token
        $accessToken = new \OAuth2\ServerBundle\Entity\AccessToken();
        $accessToken->setToken($oauth_token);
        $accessToken->setClient($client);
        $accessToken->setUserId($user_id);
        $accessToken->setExpires($expires);
        $accessToken->setScope($scope);

        // Store Access Token
        $this->em->persist($accessToken);
        $this->em->flush();
    }