JPA系列六：映射关联关系-单向多对一

1、创建实体类Order

@Table(name="JPA_ORDERS")
@Entity
public class Order {

	private Integer id;
	private String orderName;
	private Customer customer;

	@GeneratedValue
	@Id
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	@Column(name="ORDER_NAME")
	public String getOrderName() {
		return orderName;
	}

	public void setOrderName(String orderName) {
		this.orderName = orderName;
	}

	//映射单向多对一的关联关系
	//使用 @ManyToOne 来映射多对一的关联关系
	//使用 @JoinColumn 来映射外键. 
	//可使用 @ManyToOne 的 fetch 属性来修改默认的关联属性的加载策略
	@JoinColumn(name="CUSTOMER_ID")
	@ManyToOne(fetch=FetchType.LAZY)
	public Customer getCustomer() {
		return customer;
	}

	public void setCustomer(Customer customer) {
		this.customer = customer;
	}

}

2、在persistence.xml中配置

<!-- 添加持久化类 -->
<class>com.atguigu.jpa.helloworld.Order</class>

3、测试：

//保存多对一时, 建议先保存一的一端, 后保存多的一端, 这样不会多出额外的 UPDATE 语句.
@Test
public void testManyToOnePersist(){
	Customer customer = new Customer();
	customer.setAge(18);
	customer.setEmail("gg@163.com");
	customer.setLastName("GG");
	Order order1 = new Order();
	order1.setOrderName("G-GG-1");
	Order order2 = new Order();
	order2.setOrderName("G-GG-2");
	//设置关联关系
	order1.setCustomer(customer);
	order2.setCustomer(customer);
	//执行保存操作
	entityManager.persist(order1);
	entityManager.persist(order2);
	entityManager.persist(customer);
}

//默认情况下, 使用左外连接的方式来获取多的一端的对象和其关联的一的一端的对象. 
//可使用 @ManyToOne 的 fetch 属性来修改默认的关联属性的加载策略
@Test
public void testManyToOneFind(){
	Order order = entityManager.find(Order.class, 1);
	System.out.println(order.getOrderName());
	System.out.println(order.getCustomer().getLastName());
}

//不能直接删除一的一端, 因为有外键约束.
@Test
public void testManyToOneRemove(){
	Customer customer = entityManager.find(Customer.class, 7);
	entityManager.remove(customer);
}

ERROR: Cannot delete or update a parent row: a foreign key constraint fails (`jpa`.`jpa_orders`, CONSTRAINT `FK_85wpb7frm8305lho66wj458mk` FOREIGN KEY (`CUSTOMER_ID`) REFERENCES `jpa_cutomers` (`id`))

@Test
public void testManyToOneUpdate(){
	Order order = entityManager.find(Order.class, 2);
	order.getCustomer().setLastName("FFF");
}