JPQL如果不为null，则进行抓取

Question

有两个实体：职位和员工。

目标是让一个有或没有职位的员工。如果一个职位存在，则此查询有效，但如果不存在，则将引发错误。

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

找到了解决方案。如果您使用fetch Lazy，则代理将为您工作！

Spring数据将无法工作，并会引发错误：可选员工= repository.findById（id）;

效果很好。

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

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

Answer 2

这是因为您使用内部联接。像这样使用左联接：

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