public void Add(Customer customer)
        {
            ISession session = _sessionManager.GetSession();
            ITransaction transaction = session.BeginTransaction();

            try
            {
                session.Save(customer);
                session.Flush();//清除一级缓存
                
                transaction.Commit();
            }
            catch (Exception)
            {
                transaction.Rollback();
            }
            finally
            {
                session.Close();
            }
        }

       [TestMethod]
       [ExpectedException(typeof(NHibernate.Exceptions.GenericADOException))]
       public void TestAddTransation()
       {
           CustomerService customerService = new CustomerService();
           OrderService orderService = new OrderService();

           Customer customer1 = new Customer()
           { 
               FirstName = "Test",
               LastName = "TestAddTransation1",
               Age = 10
           };
           Order order1 = new Order()
           {
               OrderDate = DateTime.Now.AddMinutes(1),
               Customer = customer1
           };
           Order order2 = new Order()
           {
               OrderDate = DateTime.Now.AddMinutes(2),
               Customer = customer1
           };

           Customer customer2= new Customer()
           {
               FirstName = null,  //数据库要求不能为null，现在故意引发添加异常
               LastName = "TestAddTransation",
               Age = 10
           };
           Order order3 = new Order()
           {
               OrderDate = DateTime.Now.AddMinutes(3),
               Customer = customer2
           };
           Order order4 = new Order()
           {
               OrderDate = DateTime.Now.AddMinutes(4),
               Customer = customer2
           };

           customer1.Orders.Add(order1);
           customer1.Orders.Add(order2);
           customer2.Orders.Add(order3);
           customer2.Orders.Add(order4);

           customerService.Add(customer1); //customer1及其Order1与Order2能成功插入，因为他们与Customer2不在同一个事务
           customerService.Add(customer2); //Customer2插入失败，Order3和Order4也不能成功插入。

           Assert.IsNull(customerService.Get(customer1.CustomerId));
           Assert.IsNull(orderService.Find(order1.OrderId));
           Assert.IsNull(orderService.Find(order2.OrderId));
       }