Hibernate：删除一对多/多对一失败

Question

假设我们有三个实体：Course、Student 和 Guide。 许多学生可以访问许多课程，许多课程可以包含许多学生。这就是为什么我在Student 和Course 之间创建了bidirectional Many-To-Many-Relationship：

@Entity()
@Getter
@Setter
@ToString(exclude = "courses")
@Builder
@Table(name = "students")
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column
    private String name;

    @ManyToMany(cascade = CascadeType.REMOVE)
    @JoinTable(
            name = "courses_students",
            joinColumns = @JoinColumn(name = "students_id"),
            inverseJoinColumns = @JoinColumn(name = "courses_id"))
    private Set<Course> courses;


}

@Entity
@Builder
@Getter
@Setter
@ToString(exclude = "guide")
@Table(name="courses")
public class Course {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne()
    @JoinColumn(name="guide_id")
    private Guide guide;

    @Column
    private String title;

    @ManyToMany(mappedBy = "courses")
    Set<Student> students;
}

此外，每门课程只有一位指南，但一位指南可以教授多门课程。所以在Course和Guide之间存在birectional One-to-Many/Many-to-One-Relationship，所有权在Course旁边：

@Entity
@Table(name = "guides")
@Getter
@Setter
@Builder
public class Guide {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column
    private String name;

    @OneToMany(mappedBy = "guide")
    private List<Course> courses = new ArrayList<>();

}

当我删除所有学生时，Cascade.Remove 所有Course-entities 也会被删除，这很好。但是当我从GuideRepository 检索Guide-instances 并调用<guideInstance>.getCourses() 时，我得到了一个课程列表。我希望得到一个空列表，因为从CourseRepository 我什么也得不到！这里有什么问题？

为了重现该问题，我发布了创建实体的代码：

 Course java= Course.builder().title("Java").build();
    Course python= Course.builder().title("Python").build();
    Course kotlin= Course.builder().title("Kotlin").build();

    Set<Course> johnCourses=new HashSet<>();
    johnCourses.add(java);
    johnCourses.add(python);

    Set<Course> jackCourses=new HashSet<>();
    jackCourses.add(java);
    jackCourses.add(python);
    jackCourses.add(kotlin);

    Set<Course> henryCourses=new HashSet<>();
    henryCourses.add(python);
    henryCourses.add(kotlin);

    List<Course> drMorrisCourses=new ArrayList<Course>();
    drMorrisCourses.add(java);
    drMorrisCourses.add(kotlin);

    List<Course> drFerrisCourses=new ArrayList<Course>();
    drFerrisCourses.add(python);

    Guide drMorris = Guide.builder().name("Dr. Morris").courses(drMorrisCourses).build();
    Guide drFerris = Guide.builder().name("Dr. Ferris").courses(drFerrisCourses).build();

    Student john= Student.builder().name("John").courses(johnCourses).build();
    Student jack= Student.builder().name("Jack").courses(jackCourses).build();
    Student henry= Student.builder().name("Henry").courses(henryCourses).build();

    Set<Student> javaStudents=new HashSet<>();
    javaStudents.add(john);
    javaStudents.add(jack);

    Set<Student> kotlinStudents=new HashSet<>();
    kotlinStudents.add(jack);
    kotlinStudents.add(henry);

    Set<Student> pythonStudents=new HashSet<>();
    pythonStudents.add(john);
    pythonStudents.add(henry);
    pythonStudents.add(jack);

    java.setGuide(drMorris);
    kotlin.setGuide(drMorris);
    python.setGuide(drFerris);

    java.setStudents(javaStudents);
    python.setStudents(pythonStudents);
    kotlin.setStudents(kotlinStudents);

    studentRepository.save(john);
    studentRepository.save(jack);
    studentRepository.save(henry);

    courseRepository.save(java);
    courseRepository.save(kotlin);
    courseRepository.save(python);

    guideRepository.save(drMorris);
    guideRepository.save(drFerris);

现在所有实体都设置好了。要删除一名学生，我可以使用以下代码：

      List<Student> allStudents = studentRepository.findAll();
//Here I choose for example first student and delete it!        
studentrepository.delete(allStudents.get(0));

我删除了第一个Student-Entity，即John，它访问了Java课程。随着John的删除，Java课程也消失了，因为Cascase.REMOVE：

List<Course> courses =courseRepository.findAll();
courses.size() // -> only 2 because Java course entity is deleted!!!!

但是如果我调用第一个指导实体，这意味着drMorris 我可以看到drMorris 仍然有Java 课程，尽管Java course-Entity 已被删除！：

List<Guide> guides=guideRepository.findAll();
guides.get(0).getCourses //<- drMorris should not have Java Course!!!!!!!!

Answer 1

这是我的解决方案：

   public void deleteStudents(){
        List<Student> all = studentRepository.findAll();
       deleteStudent(all.get(2));
     }

     public void deleteStudent(Student student) {

        Set<Course> courses = student.getCourses();

         for (Course course: courses) {
             if (course.getStudents().size() == 1 ){
                 course.getGuide().getCourses().remove(course);

                 courseRepository.delete(course);
             } else {
                 course.getStudents().remove(student);
             }
         }
         studentRepository.delete(student);
    }