public static function retrieveByPK($user_id, $role_id, PropelPDO $con = null) {
        $_instancePoolKey = serialize(array((string) $user_id, (string) $role_id));
         if (null !== ($obj = UserRolePeer::getInstanceFromPool($_instancePoolKey))) {
             return $obj;
        }

        if ($con === null) {
            $con = Propel::getConnection(UserRolePeer::DATABASE_NAME, Propel::CONNECTION_READ);
        }
        $criteria = new Criteria(UserRolePeer::DATABASE_NAME);
        $criteria->add(UserRolePeer::USER_ID, $user_id);
        $criteria->add(UserRolePeer::ROLE_ID, $role_id);
        $v = UserRolePeer::doSelect($criteria, $con);

        return !empty($v) ? $v[0] : null;
    }