Question

我正在研究jpa中的映射并阅读有关ManyToOne和OneToOne映射的内容。场景是

许多员工都属于某个部门。即员工和部门之间的ManyToOne
一名员工与一张桌子相关联。即员工和办公桌之间的OneToOne。

我已将这些字段映射如下

@ManyToOne
@JoinColumn(name = "iddepartment")
private Department department;

@OneToOne
@JoinColumn(name = "iddesk")
private Desk desk;

根据Mastering Java Persistence API一书，OneToOne映射作为唯一的外键。因此，在我的示例中，一个员工记录应该分配特定的桌面ID。但即使我创建了多个员工并为他们分配了相同的服务台，它也会被插入到数据库中。那么ManyToOne和OneToOne之间的区别是什么？据我所知，他们的行为方式相同。

我坚持实体如下

/* Create EntityManagerFactory */
    EntityManagerFactory emf = Persistence
            .createEntityManagerFactory("JPAExamples");

    /* Create EntityManager */
    EntityManager em = emf.createEntityManager();
    EntityTransaction transaction = em.getTransaction();
    transaction.begin();

    Employee employee;

    employee = new Employee();
    employee.setFirstname("pranil");
    employee.setLastname("gilda");
    employee.setEmail("sdfsdf");

    Department department = em.find(Department.class, 1); // retrieves
                                                            // department
                                                            // from database
    employee.setDepartment(department);

    Desk desk = em.find(Desk.class, 1); // retrieves desk from database
    employee.setDesk(desk);

    em.persist(employee);

    transaction.commit();

    employee = new Employee();
    employee.setFirstname("prasad");
    employee.setLastname("kharkar");
    employee.setEmail("dsfsdf");
    /* Now setting the same department for another employee which is fine because 
     * there is ManyToOne relationship betweem employee and department */
    employee.setDepartment(department); 

    /*
     * Here is my doubt. Even if I set the same desk for another employee. THe insertion 
     * is fine. Shouldn't the OneToOne relationship between Employee and Desk restrict employee
     * table having duplicate foreign key for both employees? 
     * 
     * If that is not true, then what is the difference between ManyToOne and OneToOne in this case?
     * */
    employee.setDesk(desk);

    transaction.begin();
    em.persist(employee);
    transaction.commit();

注意：我不是从实体生成表格。

所以我的问题是：

JPA OneToOne注释不会在employee表上放置唯一的外键约束。这是预期的结果吗？
如果这是预期的，那么为什么ManyToOne和OneToOne关系之间没有任何区别？

编辑：根据提供的答案，我将其创建为双向关系。它给出了相同的结果。另外，我还添加了unique = true。这没有任何区别

在课堂课程

@OneToOne(mappedBy = "desk")
private Employee employee;

并在Employee类

中

@OneToOne
@JoinColumn(name = "iddesk", unique = true)
private Desk desk;

我错过了一些严肃的事吗？

Answer 1

您有一个单向的@OneToOne映射。如果您在“拥有”端设置了mappedBy属性的双向@OneToOne映射，则行为将符合预期。

E.g。在Employee对象中

@OneToOne(mappedBy="employee")
Desk desk

在Desk对象中

@OneToOne
Employee employee

jpa中一对一映射的混淆

1 个答案: