JPA-映射-(@ManyToOne)单向多对一

多个订单对应一个用户

用户类：Customer

import lombok.Data;

import javax.persistence.*;

@Data
@Entity(name = "ManyToOne_CUTOMERS")
@Table(name = "ManyToOne_CUTOMERS")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "LAST_NAME", length = 50, nullable = false)
    private String lastName;

    private String email;
}

订单类：Order

import lombok.Data;

import javax.persistence.*;

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

    @Id
    @GeneratedValue
    private Integer id;

    @Column(name = "ORDER_NAME")
    private String orderName;

    // 映射单向 n-1 的关联关系
    // 使用 @JoinColumn 来映射外键
    // 使用 @ManyToOne 来映射多对一的关联关系
    // 可使用 @ManyToOne 的 fetch 属性来修改默认的关联属性的加载策略，在左外连接和两条SQL间切换
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "CUSTOMER_ID")
    private Customer customer;
}

测试

建表

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class JPAyingshe {
    private EntityManagerFactory entityManagerFactory;
    private EntityManager entityManager;
    private EntityTransaction transaction;

    @Before
    public void init() {
        entityManagerFactory = Persistence.createEntityManagerFactory("jpaname");
        entityManager = entityManagerFactory.createEntityManager();
        transaction = entityManager.getTransaction();
        transaction.begin();
    }

    @After
    public void destroy() {
        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    @Test
    public void createTable() {}
}

添加

/**
 * 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.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);
}

查询

//默认情况下, 使用左外连接的方式来获取 n 的一端的对象和其关联的 1 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, 12);
    System.out.println(order.getOrderName());

    System.out.println(order.getCustomer().getLastName());
}

使用懒加载

// 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;
}

删除

// 不能直接删除 1 的一端, 有外键约束
@Test
public void testManyToOneRemove() {
    Order order = entityManager.find(Order.class, 12);
    entityManager.remove(order);

    Customer customer = entityManager.find(Customer.class, 11);
    entityManager.remove(customer);
}

修改

@Test
public void testManyToOneUpdate(){
    Order order = entityManager.find(Order.class, 12);
    order.getCustomer().setLastName("FFF");
}