JPA单项多对一关联关系@ManyToOne

Love The Way You Lie 2022-05-20 07:40 255阅读 0赞

# 多对一关联关系 #

package com.jpa.helloworld;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.FetchType;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.ManyToOne;
    import javax.persistence.Table;
    
    @Table(name="JPA_ORDER")
    @Entity
    public class Order {
    
    	private Integer oId;
    
    	private String oName;
    
    	private Customer customer;
    
    	@Id
    	@Column(name="JPA_ID")
    	@GeneratedValue(strategy=GenerationType.AUTO)
    	public Integer getoId() {
    		return oId;
    	}
    
    	public void setoId(Integer oId) {
    		this.oId = oId;
    	}
    
    	@Column(name="O_NAME")
    	public String getoName() {
    		return oName;
    	}
    
    	public void setoName(String oName) {
    		this.oName = oName;
    	}
    
    	//映射单向多对一关联关系 //使用@ManyToOne注解 映射多对一关联关系 //使用@JoinColumn 来映射外键 //这里的ManyToOne的fetch属性可以配置加载加载策略，有EAGER和LAZY @JoinColumn(name="CUSTOMER_ID") @ManyToOne(targetEntity=Customer.class,fetch=FetchType.EAGER)
    	public Customer getCustomer() {
    		return customer;
    	}
    
    	public void setCustomer(Customer customer) {
    		this.customer = customer;
    	}
    
    	@Override
    	public String toString() {
    		return "Order [oId=" + oId + ", oName=" + oName + ", customer=" + customer + "]";
    	}
    
    }

@Test
    	        public void testManyToOne() {
    		
    		Customer customer = new Customer();
    		customer.setName("XXX");
    		customer.setBirth(new Date());
    		customer.setEmail("123@163.com");
    		customer.setCreateDate(DateTime.now());
    		customer.setLastUpdateDate(LocalDate.now());
    		
    		Order order1 = new Order();
    		Order order2 = new Order();
    		order1.setoName("CC");
    		order2.setoName("DD");
    		order1.setCustomer(customer);
    		order2.setCustomer(customer);
    		
    		em.persist(order2);
    		em.persist(order1);
    		em.persist(customer);
    	}

**保存**

建议在使用多对一关联关系保存对象时先保存一的一端

先保存一的一端：发送三条insert语句

先保存多的一端：发送三条insert语句和两条update语句

原因：当先保存多的一端时，多的一端的关联属性为null，当再保存一的一端时，会再次发送两条update语句来更新多的一端

性能上先保存一的一端要由于先保存多的一端

**删除**

删除时不能直接删除一的一端，可以删除多的一端

**查询并修改一的一端的属性**

Order order = em.find(Order.class, 4);
    order.getCustomer().setAge(23);