jpa中一对一映射的混淆

Question

我正在学习 jpa 中的映射并阅读有关 ManyToOne 和 OneToOne 映射的信息。场景是

• 许多员工可以属于一个部门。即员工和部门之间的多对一
• 一名员工与一张办公桌相关联。即员工和办公桌之间的 OneToOne。

我已经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 映射用作唯一外键。因此，在我的示例中，应为一个员工记录分配特定的办公桌 ID。但是，即使我创建了多个员工并为他们分配了同一张办公桌，它也会被插入到数据库中。那么这里的 ManyToOne 和 OneToOne 有什么区别呢？根据我的说法，他们的行为方式相同。

我将实体持久化如下

/* 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。

所以我的问题是：

• JPA OneToOne 批注没有将唯一外键约束放在员工表上。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

课桌课

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

在 Employee 类中

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

我错过了什么严重的事情吗？

Answer 1

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

例如在 Employee 对象中

@OneToOne(mappedBy="employee")
Desk desk

在 Desk 对象中

@OneToOne
Employee employee