I changed this method to be :
    model.saveAccessToken = function (accessToken, clientId, userId, expires, callback) {
    	console.log('in saveAccessToken (accessToken: ' + accessToken + ', clientId: ' + clientId + ', userId: ' + userId + ', expires: ' + expires + ')');
	
	var accessToken = new OAuthAccessTokensModel({
		access_token: accessToken,
		client_id: clientId,
		user_id: userId,
		expires: expires
	});

	accessToken.save(callback);
    };