Hibernate 5 Java双向oneToMany字段为空，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。并且 Employee 有一个外勤部门。我可以创建员工并将他们添加到部门内的列表中。数据库表在持久化时按预期填充。如果我查询部门，我会得到部门并且员工列表已填写。这种方式一切都很好。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，如果我在查询的员工上使用 getDepartment，我会得到空值。如果我在查询的部门上使用 getEmployees，我会得到所有员工。

我尝试了这里描述的两种方法：https://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html/ch07.html#collections-bidirectional

示例 7.21。双向一对多，多对一作为关联所有者

和

示例 7.22。以一对多作为所有者的双向关联

对我来说同样的结果。

我错过了什么？

这里是完整的测试项目：updated Project zip

已解决的固定项目：SOLVED PROBLEM PROJECT

Answer 1

看起来像Owning entity 问题，所以我认为您的测试以两种不同的方式持久化数据。在您的注释@OneToMany(cascade = CascadeType.ALL) 中，您已将Department 声明为关系的所有者。所以，如果你用

持久化数据

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=?

但如果你坚持

emp.setDepartment(dept);

那么员工的部门（id）字段将不会被更新。

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

如果员工的部门 id 未持久化，则无法检索部门。如果您将 Employee 设置为关系的所有者，则效率更高，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;

并在保持关系时设置员工的部门。然后插入是用 departmentid 完成的，而不是单独更新。

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

条件代码没有明显错误，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=?

如果您添加特定的 Fetch，那么它将在单个 SQL 语句中完成。

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，您的关系映射不正确！尝试像这样更改代码。

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


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

Answer 3

用下面的代码试试，把连接表从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

如果我在持久化后关闭会话并打开一个新会话，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，以防万一有人被迭代。