private String getOpponent(EntityManager manager, String username)
			throws Exception {
		if ("ai".equalsIgnoreCase(username)) {
			return username;
		}
		if (username.equalsIgnoreCase("all"))
			return null;
		ChessUser opponent = DB_Util.findUserByUserName(manager, username);
		if (opponent == null) {
			throw new Exception("No user exist with username '" + username
					+ "'");
		}
		return opponent.getMobileHash();
	}