我在 jpa 存储库中的多对多关系中存在问题，以便在连接表中保存参考答案

【问题标题】：I have problem in many to many relationship in jpa repository for saving with reference in join table我在 jpa 存储库中的多对多关系中存在问题，以便在连接表中保存参考
【发布时间】：2020-02-19 20:33:20
【问题描述】：

我有学生课

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @Column(name = "first_name")
    private String firstName;

    @ManyToMany(mappedBy = "students", fetch = FetchType.LAZY, cascade = {CascadeType.MERGE, CascadeType.DETACH, CascadeType.PERSIST, CascadeType.REFRESH})
    private List<Course> courses;

    public void addCourse(Course theCourse) {
        if (courses == null) {
            courses = new ArrayList<>();
        }
        courses.add(theCourse);
        theCourse.addStudents(this);
    }

}

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;

    @ManyToMany(fetch = FetchType.LAZY, cascade = {CascadeType.MERGE, CascadeType.PERSIST, CascadeType.PERSIST, CascadeType.REFRESH})
    private List<Student> students;

    public void addStudents(Student theStudent) {
        if (students == null) {
            students = new ArrayList<>();
        }
        students.add(theStudent);
    }
}

//definitely I wrote syntax wrong here because i don't copy and paste from code.
Course c =new Course(//fill constrouctor);
CourseRepositoryy.save(c);

student s = new Student(//fill constrouctor);
s.addCourse(c)
StudentRepository.save(s);

or

student s = new Student(//fill constrouctor);
Course c =new Course(//fill constrouctor);
c.addStudents(s);
save(s);
save(c);

现在我想创建学生和课程并将它们保存在连接表中保存每个参考。我不知道先是哪个，然后再添加另一个。有人可以帮助我并提供描述该主题的参考资料吗？

【问题讨论】：

标签： hibernate jpa spring-data-jpa repository spring-repositories

【解决方案1】：

如果您想要多对多双向关系，您应该定义拥有方。因此，如果您选择 Course 作为拥有方，则映射将如下所示：

public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;

    @ManyToMany(cascade = CascadeType.PERSIST)
    private List<Student> students = new ArraysList<>();
...

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @Column(name = "first_name")
    private String firstName;

    @ManyToMany(mappedBy = "students")
    private List<Course> courses = new ArrayList<>();

来自@ManyToManylink的javadoc

如果关联是双向的，则一方可以指定为拥有方。如果关系是双向，非拥有方必须使用 mappedBy 元素 ManyToMany 注释指定关系字段或拥有方的财产。

如果你有如上的映射：Stundent的保存操作会级联到Course。

Course course = new Course();
List<Student> studentList = new ArrayList<>();

course.getStudents().addAll(studentList);

em.persist(course);

更多详情：official docs

【讨论】：