Hibernate--一对多/多对一

    一对多关联映射：
        在多的一端添加一个外键指向一的一端，它维护的关系是一指向多
    多对一关联映射：
        咋多的一端加入一个外键指向一的一端，它维护的关系是多指向一

　　在配置文件中添加：
   　　 在一一端使用<set><key></key><one-to-many></one-to-many></set>
       　　 <key>指定的外键字段必须和<many-to-one>指定的外键字段一致
  　　 在多一端使用<many-to-one>

public class Customer {

    private Integer id;
    private String name;
    private Character gender;
    private Integer age;
    private String level;

    //一对多
    private Set<Order> orders = new HashSet<Order>();
    
    public Customer() {
        super();
        
    }

　　getters and setters
｝

public class Order {

    private Integer id;
    private String orderno;
    private String productName;
    
    //多对一
    private Customer customer;

    public Order() {
        super();
        // TODO Auto-generated constructor stub
    }

　　getters and setters
} 

<hibernate-mapping>
    <class name="com.roxy.hibernate.pojo.Customer" table="t_customer">
    
        <id name="id" column="c_id">
            <generator class="native"></generator>
        </id>
        
        <property name="name" column="c_name" not-null="true"></property>
        <property name="gender" column="c_gender" length="1"></property>
        <property name="age" column="c_age"></property>
        <property name="level" column="c_level" length="20"></property>
        
        <set name="orders">
            <key column="customer_id"></key>
            <one-to-many class="com.roxy.hibernate.pojo.Order"/>
        </set>
    </class>
</hibernate-mapping>

<hibernate-mapping package="com.roxy.hibernate.pojo">

    <!-- 类和表的映射 -->
    <class name="Order" table="t_order">
        <id name="id" column="id">
            <generator class="native"></generator>
        </id>
        <!-- 其他属性映射 -->
        <property name="orderno" column="orderno" length="20"></property>
        <property name="productName" column="product_name" length="100"></property>

        <!-- 多对一 -->
        <many-to-one name="customer" class="Customer" column="customer_id" />
    
    </class>
</hibernate-mapping>   

        <!-- mapping文件 -->
        <mapping resource="com/roxy/hibernate/pojo/Customer.hbm.xml"/>
        <mapping resource="com/roxy/hibernate/pojo/Order.hbm.xml"/>

Hibernate: 
       alter table t_order 
       add constraint FKesy3n2gc3fa0s3trrk3tvyv9a 
       foreign key (customer_id) 
       references t_customer (c_id)

----- hibernate首先为t_order和t_customer添加外键关联

Hibernate: 
    insert 
    into
        t_customer
        (c_name, c_gender, c_age, c_level) 
    values
        (?, ?, ?, ?)

----- 向t_customer中插入数据（session.save(cust);）

Hibernate: 
    insert 
    into
        t_order
        (orderno, product_name, customer_id) 
    values
        (?, ?, ?)

----- 向t_order中插入数据（session.save(o1);）

Hibernate: 
    insert 
    into
        t_order
        (orderno, product_name, customer_id) 
    values
        (?, ?, ?)

----- 向t_order中插入数据（session.save(o2);）

Hibernate: 
    update
        t_order 
    set
        customer_id=? 
    where
        id=?

----- 维护两个表之间的关系，hibernate更新t_customer数据

Hibernate: 
    update
        t_order 
    set
        customer_id=? 
    where
        id=?

----- 维护两个表之间的关系，hibernate更新t_customer数据