Hibernate关联关系之——单向n-1

1 .单向 n-1 关联只需从n的一端可以访问1的一端

2.域模型: 从Order到Customer的多对一单向关联需要在Order类中定义一个Customer属性,而在Customer类中无需定义存放Order对象的集合属性

      

3.关系数据模型:ORDERS表中的CUSTOMER_ID参照CUSTOMER表的主键

测试代码：

1.Customer实体类和Order实体类

package com.elgin.hibernate.nto1;

public class Customer {
   
	private int customerId;
	private String customerName;
	public int getCustomerId() {
		return customerId;
	}
	public void setCustomerId(int customerId) {
		this.customerId = customerId;
	}
	public String getCustomerName() {
		return customerName;
	}
	public void setCustomerName(String customerName) {
		this.customerName = customerName;
	}
		
}

package com.elgin.hibernate.nto1;

public class Order {
   
	private int orderId;
	private String orderName;
	private Customer customer;
	public int getOrderId() {
		return orderId;
	}
	public void setOrderId(int orderId) {
		this.orderId = orderId;
	}
	public String getOrderName() {
		return orderName;
	}
	public void setOrderName(String orderName) {
		this.orderName = orderName;
	}
	public Customer getCustomer() {
		return customer;
	}
	public void setCustomer(Customer customer) {
		this.customer = customer;
	}
	
	
}

2.2个实体类分别对应的hbm映射文件

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 2015-9-21 20:58:02 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping>
    <class name="com.elgin.hibernate.nto1.Customer" table="CUSTOMERS">
        <id name="customerId" type="int">
            <column name="CUSTOMER_ID" />
            <generator class="native" />
        </id>
        <property name="customerName" type="java.lang.String">
            <column name="CUSTOMER_NAME" />
        </property>
    </class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 2015-9-21 20:58:02 by Hibernate Tools 3.4.0.CR1 -->
<hibernate-mapping package="com.elgin.hibernate.nto1">
    <class name="Order" table="ORDERS">
        <id name="orderId" type="int">
            <column name="ORDER_ID" />
            <generator class="native" />
        </id>
        <property name="orderName" type="java.lang.String">
            <column name="ORDER_NAME" />
        </property>
        <!-- 
          映射多对一关联关系 使用many-to-one映射多对一关联关系
          name：多这一端关联一的那一端的属性名
          class：一那一端对应的类名
          column：一那一端对应多的一端对应的数据表中字段的名字                         
          -->
        <many-to-one name="customer" class="Customer">
            <column name="CUSTOMER_ID" />
        </many-to-one>
    </class>
</hibernate-mapping>

3.Junit单元测试类

        @Test
	public void testMany2OneSave(){
		Customer customer=new Customer();
		customer.setCustomerName("CC");
		
		Order order1=new Order();
		order1.setOrderName("order-5");
		order1.setCustomer(customer);
		
		Order order2=new Order();
		order2.setOrderName("order-6");
		order2.setCustomer(customer);
		//发出了3条insert语句
		//先插入1的一端，再插入n的一端，只有insert语句
		/*session.save(customer);
		session.save(order1);
		session.save(order2);*/
		
		//发出了3条insert语句和2条update语句
		//先插入n的一端，再插入1的一端，要额外发出update语句，推荐使用第一种方式
		session.save(order1);
		session.save(order2);
		session.save(customer);
	}
	
	@Test
	public void testMany2OneGet(){
		//1.若查询的n的那一端的对象，默认情况下，只查询n的一端的对象，
		//2.它所关联的1的那一端的对象为代理对象，等到使用的时候才会发出sql语句查询对此代理对象进行初始化
		//3.如果在使用此对象时session已关闭，则会抛出懒加载异常LazyInitializationException
		Order order=(Order) session.get(Order.class, 1);
		System.out.println(order.getOrderName());
		System.out.println(order.getCustomer().getCustomerName());
	}
	@Test
	public void testMany2OneDelete(){
		//在不设定关联关系的前提下，如果1的一端有对象被n一端的对象引用，不能直接删除1一端的对象
		Customer customer=(Customer) session.get(Customer.class, 1);
		session.delete(customer);
	}