JPQL join fetch 如果不为空

Question

有两个实体：职位和员工。

目标是让员工有或没有职位。如果位置存在，则此查询有效，但如果不存在，则会抛出错误。

如果员工有或没有职位，如何更改此查询以接收员工？

public Employee getEmployee(Long id) {
    EntityManager em = entityManagerFactory.createEntityManager();
    em.getTransaction().begin();
    Employee employee;
    try {
        employee = em.createQuery("select e " +
                "from Employee e " +
                "join fetch e.position p " +
                "where e.employeeId= :id)", Employee.class)
                .setParameter("id", id)
                .getSingleResult();

        em.getTransaction().commit();
    } catch (NoResultException ex){
        throw new RuntimeException("MAMA mia");
    }
    return employee;
}


 @Entity
 @Table(name = "position")
 public class Position {

     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "POSITION_ID", updatable = false)
     private Long id;

     @Column(name = "NAME")
     private String name;

     @JoinTable(name = "Position_Employee_JT")
     @OneToMany(fetch = LAZY, cascade = ALL)
     private Set<Employee employeeSet = new HashSet<();
 }



 @Entity
 @Table(name = "employee")
 public class Employee {

     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
     @Column(name = "EMPLOYEE_ID", updatable = false)
     private Long employeeId;

     @ManyToOne(fetch = LAZY)
     private Position position;
}

Answer 1

找到解决办法。如果您使用 fetch Lazy，代理将为您工作！

Spring 数据将无法工作并会抛出错误：可选员工 = repository.findById(id);

它工作正常。

public Employee getEmployee(Long id) {
    EntityManager em = entityManagerFactory.createEntityManager();
    em.getTransaction().begin();
    Employee employee = em.find(Employee.class,id);
    em.getTransaction().commit();
    return employee;
}

此处说明： https://vladmihalcea.com/how-does-a-jpa-proxy-work-and-how-to-unproxy-it-with-hibernate/

Answer 2

这是因为您使用了内部联接。像这样使用左连接：

    employee = em.createQuery("select e " +
            "from Employee e " +
            "left join fetch e.position p " +
            "where e.employeeId= :id)", Employee.class)
            .setParameter("id", id)
            .getSingleResult();

JPQL join fetch 如果不为空

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hibernate

jpa

jpql