JPA中实现单向多对一的关联关系

港控/mmm° 2023-10-08 10:43 13阅读 0赞

## 场景 ##

JPA入门简介与搭建HelloWorld(附代码下载)：

[https://blog.csdn.net/BADAO\_LIUMANG\_QIZHI/article/details/103473937][https_blog.csdn.net_BADAO_LIUMANG_QIZHI_article_details_103473937]

在上面博客中搭建好JPA的HelloWorld后，只是对customer实体类完成数据库的映射。

若要实现单向多对一的映射关系，怎样实现。

注：

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[https://blog.csdn.net/badao\_liumang\_qizhi][https_blog.csdn.net_badao_liumang_qizhi]  
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## 实现 ##

数据库中新建JPA\_ORDER表

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0JBREFPX0xJVU1BTkdfUUlaSEk_size_16_color_FFFFFF_t_70][]

其中一个用户可以有多个订单，但是一个订单只能属于一个用户，所以形成了单向多对一的关系。

订单表中CUSTOMER\_ID就是与用户表相关联的外键。

在包下新建实体类Order

package com.badao.jpa.helloworld;
    
    import javax.persistence.Column;
    import javax.persistence.Entity;
    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_ORDERS")
    @Entity
    public class Order {
     private Integer id;
     private String orderName;
    
     private Customer customer;
    
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @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 n-1 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
     public Customer getCustomer() {
      return customer;
     }
    
     public void setCustomer(Customer customer) {
      this.customer = customer;
     }
    }

主要是通过@ManyToOne来映射多对一的关联关系，使用@JoinColumn来映射主键。

然后在配置文件persistence.xml中添加实体类Order的配置

<class>com.badao.jpa.helloworld.Order</class>

添加位置如下

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0JBREFPX0xJVU1BTkdfUUlaSEk_size_16_color_FFFFFF_t_70 1][]

继续上面那篇博客的单元测试类。继续进行编写测试方法

/**
       * 保存多对一时, 建议先保存 1 的一端, 后保存 n 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(customer);
       entityManager.persist(order1);
       entityManager.persist(order2);
       
       
      }

执行成功后查看数据库

用户表

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0JBREFPX0xJVU1BTkdfUUlaSEk_size_16_color_FFFFFF_t_70 2][]

订单表

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0JBREFPX0xJVU1BTkdfUUlaSEk_size_16_color_FFFFFF_t_70 3][]

[https_blog.csdn.net_BADAO_LIUMANG_QIZHI_article_details_103473937]: https://blog.csdn.net/BADAO_LIUMANG_QIZHI/article/details/103473937
[https_blog.csdn.net_badao_liumang_qizhi]: https://blog.csdn.net/badao_liumang_qizhi
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