获取正确的查询以获取多对多春季jpa关系的学生课程

问题描述

任何人都可以帮助正确的JPQL查询来获得学生的课程吗？

@Entity
@Table(name = "students")
public class Student implements Serializable {

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

...

@ManyToMany(fetch = FetchType.LAZY,cascade = CascadeType.PERSIST)
@JoinTable(name = "students_courses",joinColumns = {
                @JoinColumn(name = "student_id",referencedColumnName = "id",nullable = false,updatable = false)},inverseJoinColumns = {
                @JoinColumn(name = "course_id",updatable = false)})
private Set<Course> courses = new HashSet<>();

...

和

@Entity
@Table(name = "courses")
public class Course implements Serializable {

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

...

@ManyToMany(mappedBy = "courses",fetch = FetchType.LAZY)
private Set<Student> students = new HashSet<>();

...

在学生存储库中，我创建了以下JPQL查询：

public interface StudentRepository extends CrudRepository<Student,Long> {

    /*THE QUERY SHOULD GO HERE */
    @Query("")
    List<Course> getStudentCourses(Long studentId);
}

我从StudentController打电话给我

@RestController
@RequestMapping("/student")
public class StudentController {

    @Autowired
    public StudentRepository studentRepository;

    @GetMapping("/{id}/courses")
    public List<Course> getStudentCourses(@PathVariable("id") Long id){
       return studentRepository.getStudentCourses(id);
    }

我尝试了很多查询，但是，我收到以下邮递员回复：

[
    {
        "id": 1,"title": "Machine Learning","abbreviation": "ML","modules": 12,"fee": 1500.0,"students": [
            {
                "id": 1,"name": "John Doe","age": 15,"grade": "8th","courses": [
                    {
                        "id": 1,"students": [
                            {
                                "id": 1,"courses": [
                                    {
                                      .......

我猜我的查询有误，我们将不胜感激。

解决方法

这可能是由于双向映射导致无限递归而发生的。 您可以使用 @JsonManagedReference 和 @JsonBackReference 与实体 在您的情况下使用 @JsonManagedReference 与学生实体和 @JsonBackReference 与课程实体一起使用，它将在尝试对结果进行序列化时防止循环

查询将是这样

"select s.courses from Student s join s.courses where s.id = :id "