Hibernate选择插入的值然后插入连接表

Question

这是显示我的问题的虚拟代码示例。

我有两个与ManyToMany有关系的实体。

@Entity
public class Class implements Serializable {

  private static final long serialVersionUID = 2721854845989429844L;

  @Id
  @Type(type = "uuid-char")
  private UUID id;

  private String name;

  public UUID getId() {
    return id;
  }

  public void setId(UUID id) {
    this.id = id;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

@Entity
public class Student implements Serializable {

  private static final long serialVersionUID = -1203104744560316339L;

  @Id
  @Type(type = "uuid-char")
  private UUID id;

  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Integer number;

  private String name;

  @ManyToMany
  @JoinTable(name = "student_class",
      joinColumns = @JoinColumn(name = "student_number", referencedColumnName = "number"),
      inverseJoinColumns = @JoinColumn(name = "class_id", referencedColumnName = "id"))
  private Set<Class> classes;

  public UUID getId() {
    return id;
  }

  public void setId(UUID id) {
    this.id = id;
  }

  public Integer getNumber() {
    return number;
  }

  public void setNumber(Integer number) {
    this.number = number;
  }

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }

  public Set<Class> getClasses() {
    return classes;
  }

  public void setClasses(Set<Class> classes) {
    this.classes = classes;
  }
}

Join Table以其编号引用Student，该编号由MySQL自动生成，并通过其id引用Class。

当我尝试插入带有类的Student时，hibernate尝试在student表中插入行后直接在student_class中插入记录，但是在插入后生成学号，然后抛出异常，该列student_number不能为null。

如何在连接表中插入之前告诉hibernate在连接列中选择生成的值？

包含逻辑的类具有以下代码。 其中classRepository和studentRepository是扩展spring CrudRepository的接口。其中没有自定义代码。

    Class class1 = new Class();
    class1.setId(UUID.randomUUID());
    class1.setName("Math");
    classRepository.save(class1);

    Class class2 = new Class();
    class2.setId(UUID.randomUUID());
    class2.setName("Chemistry");
    classRepository.save(class2);

    Set<Class> classes = new HashSet<>();
    classes.add(class1);
    classes.add(class2);

    Student student = new Student();
    student.setId(UUID.randomUUID());
    student.setName("Student 1");
     student.setClasses(classes);

    studentRepository.save(student);

控制台输出：

Hibernate: 
    /* load com.perksCamp.hello.Class */ select
        class0_.id as id1_0_0_,
        class0_.name as name2_0_0_ 
    from
        class class0_ 
    where
        class0_.id=?
Hibernate: 
    /* insert com.perksCamp.hello.Class
        */ insert 
        into
            class
            (name, id) 
        values
            (?, ?)
Hibernate: 
    /* load com.perksCamp.hello.Class */ select
        class0_.id as id1_0_0_,
        class0_.name as name2_0_0_ 
    from
        class class0_ 
    where
        class0_.id=?
Hibernate: 
    /* insert com.perksCamp.hello.Class
        */ insert 
        into
            class
            (name, id) 
        values
            (?, ?)
Hibernate: 
    /* load com.perksCamp.hello.Student */ select
        student0_.id as id1_1_0_,
        student0_.name as name2_1_0_,
        student0_.number as number3_1_0_ 
    from
        student student0_ 
    where
        student0_.id=?
Hibernate: 
    /* insert com.perksCamp.hello.Student
        */ insert 
        into
            student
            (name, number, id) 
        values
            (?, ?, ?)
Hibernate: 
    /* insert collection
        row com.perksCamp.hello.Student.classes */ insert 
        into
            student_class
            (student_number, class_id) 
        values
            (?, ?)
2018-04-07 12:42:44.268  WARN 5508 --- [nio-8080-exec-2] o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 1048, SQLState: 23000
2018-04-07 12:42:44.268 ERROR 5508 --- [nio-8080-exec-2] o.h.engine.jdbc.spi.SqlExceptionHelper   : Column 'student_number' cannot be null
2018-04-07 12:42:44.268  INFO 5508 --- [nio-8080-exec-2] o.h.e.j.b.internal.AbstractBatchImpl     : HHH000010: On release of batch it still contained JDBC statements
2018-04-07 12:42:44.362 ERROR 5508 --- [nio-8080-exec-2] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement] with root cause

com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'student_number' cannot be null