JPA存储库多对一

问题描述

我有 Student 实体和 Course 实体。这是 @ManyToOne 关系，即 Student 一次只能参加一门课程，但课程可能有多个学生。

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String studentId;
    private String firstName;
    private String secondName;

    @ManyToOne
    @JoinColumn(name = "course_id")
    //@JsonIgnore
    private Course course;
@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String courseName;
    @OneToMany(cascade = {CascadeType.PERSIST,CascadeType.MERGE},mappedBy = "course",orphanRemoval = true,targetEntity = Student.class)
private List<Student> students = new ArrayList<>();

我使用以下 json 发布我的数据：

    {   
        "id": 1,"courseName": "course134","students" : [
            {
                "id" : 1,"studentId": "123","firstName": "John1","secondName": "Name1"
                
            },{
                "id" : 2,"studentId": "1234567","firstName": "John2","secondName": "Name2"
                
            }

然后，当我收到课程时：

    {
        "id": 1,"students": []
    }

如何列出参加特定课程的学生？ 我在 StudentRepository 中创建了一个查询

    @Query("SELECT s from  Student s where s.id = :courseName")
        Optional<Student> getStudentByCourseName(String courseName);

还是不行。

这是我的存储库代码：

    @Repository
    public interface CourseRepository extends JpaRepository<Course,Long> {
        Optional<Course> findCourseByCourseName(String courseName);
        @Query("SELECT c.students FROM Course c WHERE c.courseName = :courseName")
        Optional<Student> getStudentsByCourseName(String courseName);
    }

这是我的服务方法

      public Optional<Student> findStudentByCourse(String courseName){
            return courseRepository.getStudentsByCourseName(courseName);
        }

最后是我的控制器：

@GetMapping("/student/course/{courseName}")
public ResponseEntity<Student> findCoursesWithStudentId(@PathVariable String courseName) {
    Optional<Student> byCourseName = studentService.findStudentByCourse(courseName);
    if (byCourseName.isPresent()) {
        return ResponseEntity.ok(byCourseName.get());
    } else {
        return ResponseEntity.notFound().build();
    }
}

解决方法

您应该查询 Course 表，而不是 Student 表。此外，查询将返回列表，而不仅仅是一个实体，因此还要更改方法的返回类型...

@Query("SELECT c.students FROM Course c WHERE c.courseName = :courseName")
List<Student> getStudentsByCourseName(String courseName) {}

编辑 你总是可以这样做：

1. 执行简单的方法：

Course findByCourseName(String courseName) {}

2. 然后通过一个简单的方法获取它的学生：

course.getStudents();