java - @OneToMany 中不必要的更新

Question

我的模型对象如下

Employee.java
@Entity
@Table(name = "EMPLOYEE")
public class Employee {
    @Id
    @SequenceGenerator(name = "emp_seq", sequenceName = "seq_employee")
    @GeneratedValue(generator = "emp_seq")
    @Column(name = "EMPLOYEE_ID")
    private Integer employeeId;
    @Column(name = "EMPLOYEE_NAME")
    private String employeeName;
}

Department.java
@Entity
@Table(name = "DEPARTMENT")
public class Department {
    @Id
    @Column(name = "DEPARTMENT_ID")
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer departmentId;
    @Column(name = "DEPARTMENT_NAME")
    private String departmentName;
    @Column(name = "LOCATION")
    private String location;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "DEPARTMENT_ID")
    private List<Employee> employees = new ArrayList<>();
}

在保存它时，它会生成两个额外的更新语句。

测试类

    Employee e1 = new Employee();
    e1.setEmployeeName("Employee-1");

    Employee e2 = new Employee();
    e2.setEmployeeName("Employee-2");

    Department d = new Department();
    d.setDepartmentName("Test");
    d.setLocation("Test");

    d.getEmployees().add(e1);
    d.getEmployees().add(e2);
    em.getTransaction().begin();
    em.persist(d);
    em.getTransaction().commit();

提交后会生成以下语句...

Hibernate: insert into DEPARTMENT (DEPARTMENT_NAME, LOCATION, DEPARTMENT_ID) values (?, ?, ?)
Hibernate: insert into EMPLOYEE (EMPLOYEE_NAME, EMPLOYEE_ID) values (?, ?)
Hibernate: insert into EMPLOYEE (EMPLOYEE_NAME, EMPLOYEE_ID) values (?, ?)
**Hibernate: update EMPLOYEE set DEPARTMENT_ID=? where EMPLOYEE_ID=?
**Hibernate: update EMPLOYEE set DEPARTMENT_ID=? where EMPLOYEE_ID=?

我的问题是为什么需要 2 个额外的更新(用 * 标记)语句？

Answer 1

这是 Hibernate 正常执行操作的顺序。看看这个

https://docs.jboss.org/hibernate/orm/4.2/javadocs/org/hibernate/event/internal/AbstractFlushingEventListener.html#performExecutions%28org.hibernate.event.spi.EventSource%29

根据此文档:

Execute all SQL (and second-level cache updates) in a special order so that foreign-key constraints cannot be violated: