Hibernate 5 Java双向oneToMany字段为null，但表包含数据

Question

我有两个实体部门和员工。部门有一份员工名单。员工有一个现场部门。我可以创建Employees并将它们添加到Department内的列表中。数据库表在persist上按预期填充。如果我查询部门，我会得到部门和员工名单。这一切都很好。如果我查询Employee并获取Department字段，则返回null。

@Entity
@Table(name = "DEPARTMENT")
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "DPT_ID")
    private long id;

    @Column(name = "NAME", nullable = false, unique = true)
    private String name;


    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name = "DEPARTMENT") //we need to duplicate the physical information
    private List<Employee> employees = new ArrayList<>();
…

--

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

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "EMP_ID")
    private long id;

    @Column(name = "NAME", nullable = false)
    private String name;

    @Column(name = "DESIGNATION")
    private String designation;

    @ManyToOne
    @JoinColumn(name = "DEPARTMENT", insertable = false, updatable = false)
    private Department department;
...

--

employee.getDepartment（）返回null的查询

        session = HibernateUtil.getSessionFactory().openSession();
        transaction = session.getTransaction();
        transaction.begin();

        Department department = new Department();
        department.setName("IT Department");

        Employee employee1 = new Employee();
        employee1.setName("Adam");
        employee1.setDesignation("Manager");

        Employee employee2 = new Employee();
        employee2.setName("Miller");
        employee2.setDesignation("Software Engineer");

        Employee employee3 = new Employee();
        employee3.setName("Smith");
        employee3.setDesignation("Associate  Engineer");

        department.getEmployees().add(employee1);
        department.getEmployees().add(employee2);
        department.getEmployees().add(employee3);

        session.persist(department);
        session.flush();
        transaction.commit();


        transaction = session.getTransaction();
        transaction.begin();

        {
            CriteriaBuilder builder = session.getCriteriaBuilder();
            CriteriaQuery<Employee> query = builder.createQuery(Employee.class);
            Root<Employee> root = query.from(Employee.class);
            query.select(root);
            Query<Employee> q = session.createQuery(query);
            List<Employee> employees = q.getResultList();
            for (Employee employee : employees) {
                System.out.println("EMPLOYEE NAME: " + employee.getName());
                System.out.println("DEPARTMENT NAME: " + employee.getDepartment()); // gives null
            }
        }
        {
            CriteriaBuilder builder = session.getCriteriaBuilder();
            CriteriaQuery<Department> query = builder.createQuery(Department.class);
            Root<Department> root = query.from(Department.class);
            query.select(root);
            Query<Department> q = session.createQuery(query);
            List<Department> departments = q.getResultList();
            for (Department deps : departments) {
                System.out.println(deps.getName());
                System.out.println(deps.getEmployees()); // list of employees is filled
            }
        }

表格似乎填写正确。但是如果我在查询的Employee上使用getDepartment，我会得到null。如果我在查询的部门使用getEmployees，我将获得所有员工。

我试过这里描述的两种方式：https://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html/ch07.html#collections-bidirectional

例7.21。双向一对多，多对一方作为协会所有者

和

例7.22。作为所有者的一对多方的双向关联

对我来说也是如此。

我错过了什么？

这是完整的测试项目：updated Project zip

已解决的固定项目：SOLVED PROBLEM PROJECT

Answer 1

看起来像Owning entity问题，所以我认为你的测试是以两种不同的方式持久化数据。在您的注释@OneToMany(cascade = CascadeType.ALL)中，您已声明Department是该关系的所有者。所以，如果你坚持使用数据

dept.getEmployees().add(emp);

然后部门（id）字段将被更新

Hibernate: insert into EMPLOYEE (EMP_ID, DESIGNATION, NAME) values (null, ?, ?)
Hibernate: update EMPLOYEE set DEPARTMENT=? where EMP_ID=?

但如果坚持下去

emp.setDepartment(dept);

那么员工的部门（id）字段将不会更新。

Hibernate: insert into EMPLOYEE (EMP_ID, DESIGNATION, NAME) values (null, ?, ?)

如果员工的部门ID未持久存在，则无法检索部门。如果您将Employee作为关系的所有者，那么它会更有效，因为它具有外键。

@OneToMany(cascade = CascadeType.ALL, mappedBy="department")
private List<Employee> employees; // don't need to make a list, only for fetches
// and 
@ManyToOne
@JoinColumn(name = "DEPARTMENT")
private Department department;

在坚持关系时设置员工的部门。然后使用departmentid完成插入，而不单独更新。

Hibernate: insert into EMPLOYEE (EMP_ID, DEPARTMENT, DESIGNATION, NAME) values (null, ?, ?, ?)

标准代码没有明显的错误，因为JPA将遵循带注释的关系，但它在两个单独的查询中这样做，因为您没有特定的连接。

Hibernate: select employee0_.EMP_ID as EMP_ID1_1_, employee0_.DEPARTMENT as DEPARTME4_1_, employee0_.DESIGNATION as DESIGNAT2_1_, employee0_.NAME as NAME3_1_ from EMPLOYEE employee0_
Hibernate: select department0_.DPT_ID as DPT_ID1_0_0_, department0_.NAME as NAME2_0_0_ from DEPARTMENT department0_ where department0_.DPT_ID=?

如果添加特定的Fetch，则它将在单个SQL语句中执行。

root.fetch("department");

和

Hibernate: select employee0_.EMP_ID as EMP_ID1_1_0_, department1_.DPT_ID as DPT_ID1_0_1_, employee0_.DEPARTMENT as DEPARTME4_1_0_, employee0_.DESIGNATION as DESIGNAT2_1_0_, employee0_.NAME as NAME3_1_0_, department1_.NAME as NAME2_0_1_ from EMPLOYEE employee0_ inner join DEPARTMENT department1_ on employee0_.DEPARTMENT=department1_.DPT_ID

Answer 2

在我看来，你的关系映射是不正确的！尝试更改这样的代码。

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


@OneToMany(mappedBy = "department",cascade = CascadeType.ALL)
private List<Employee> employees = new ArrayList<>();

Answer 3

尝试使用以下代码，只需将联接表从Employee指向Department。

@ManyToOne
@JoinColumn(name = "DPT_ID", insertable = false, updatable = false)
private Department department;

Answer 4

我发现了问题。我使用相同的会话进行持久化和查询。

session = HibernateUtil.getSessionFactory().openSession();
transaction = session.getTransaction();
transaction.begin();
…
session.persist(stuff);
session.flush();
transaction.commit();

transaction = session.getTransaction();
transaction.begin();

query stuff

如果我在持续并打开一个新会话后关闭会话，一切正常。

我在我的问题中添加了一个固定版本的测试项目，以防有人感兴趣。