Hibernate中one-to-many保存many方

　　使用Hibernate的人，对于one-to-many肯定不会陌生。最常见的做法是设置one方的inverse=”true”和cascade（xml方式。如果使用注解，那么就是mappedBy和cascade），然后在保存或更新时，仅对one方进行操作就可以级联到many方。
　　但是有时候也会有一种需求，就是one方跟many方的数据变化不是同时的。比如说，一个顾客(Customer)可以有多个订单(Order)，某个顾客可能先在系统中有了记录，然后过了一段时间，他下订单了。这个情景对应的两个类简单抽象如下：　

@Entity
@Table(name = "customer")
public class Customer {
    private int id;
    private String name;
    private Set<Order> orders = new HashSet<Order>();

    @Id
    public int getId() {return id;}
    public void setId(int id) {this.id = id;}

    @Column(length = 50)
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}

    @OneToMany(targetEntity = Order.class, cascade = CascadeType.ALL, mappedBy = "customer", orphanRemoval = true)
    public Set<Order> getOrders() {return orders;}
    public void setOrders(Set<Order> orders) {this.orders = orders;}

    public Customer(int id) {this.id = id;}
    public Customer() {}
}

@Entity
@Table(name = "orders")
public class Order {
    private int id;
    private String number;
    private Customer customer;

    @Id
    public int getId() {return id;}
    public void setId(int id) {this.id = id;}

    @Column(length = 50)
    public String getNumber() {return number;}
    public void setNumber(String number) {this.number = number;}

    @ManyToOne(targetEntity = Customer.class)
    @JoinColumn(name = "customer_id")
    public Customer getCustomer() {return customer;}
    public void setCustomer(Customer customer) {this.customer = customer;}

    public Order(int id, String number, Customer customer)
    {
        this.id = id;
        this.number = number;
        this.customer = customer;
    }

    public Order() {}
}

　　简化一下需求，假设这个顾客只下了一个订单，对应只有一个Order实例，名为order。这时候为了保存order，自然不能通过new一个Customer实例保存之，通过级联来实现保存order的方式，而应该直接保存order对象。那因为order有个customer成员变量，所以一个朴素的想法就是先通过session.get，得到Customer对象，然后setCustomer，再save。
　　的确是很朴素的想法，但是我们已经知道了customer的id（废话，要不然怎么session.get），却仅仅是为了order.setCustomer()，就得去查询customer表格。如果数据量很大，会很耗时。这明显是很违反直觉的，JDBC中也不是这么操作。
　　下面简单给出一个小示例，给出我的解决方式：

Session session = null;
Transaction transaction = null;
try
{
    session = HibernateUtil.createSession();
    transaction = session.beginTransaction();
    //已经知道了customer的id，则直接new一个Customer对象，只有id属性非空
    Order order1 = new Order(1, "orderNumber1", new Customer(1));
    //直接保存Order对象
    session.save(order1);
    transaction.commit();
}catch (Exception e)
{
    e.printStackTrace();
    if(null != transaction)
        transaction.rollback();
}finally
{
    if(null != session)
        HibernateUtil.close(session);
}

　　上述代码，Hibernate产生的SQL语句如下：
Hibernate: select customer_.id, customer_.name as name2_0_ from customer customer_ where customer_.id=?
Hibernate: insert into orders (customer_id, number, id) values (?, ?, ?)
结果它还是去查找customer表了，我大处女座如何能忍？？
　　后面发现，为主键设置了generator之后，Hibernate就不会再去查询customer表了。如下：

@Entity
@Table(name = "customer")
public class Customer {
    private int id;
    private String name;
    private Set<Order> orders = new HashSet<Order>();

    @Id
    @GenericGenerator(name = "assigned", strategy = "assigned")
    @GeneratedValue(generator = "assigned")
    public int getId() {return id;}
    //...
}

@Entity
@Table(name = "orders")
public class Order {
    private int id;
    private String number;
    private Customer customer;

    @Id
    @GenericGenerator(name = "assigned", strategy = "assigned")
    @GeneratedValue(generator = "assigned")
    public int getId() {return id;}
    //...
}

最后产生的SQL语句如下：
Hibernate: insert into orders (customer_id, number, id) values (?, ?, ?)
嗯，舒坦了！！！！！

PS：前提已经知道customer的id。如果只知道name，那还是要去查询customer表，这是避不开的。