Spring boot SpringBoot：如何只与给定id的实体建立多对多关系？

我有两个表（考试和部门）和一些填充数据，它们之间存在多对多关系

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

    @NotBlank
    @Size(min = 5)
    private String name;

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "exam_department", 
        joinColumns = @JoinColumn(name = "exam_id"), 
        inverseJoinColumns = @JoinColumn(name = "department_id")) 
    @JsonIgnoreProperties("exams")  
    private Set<Department> departments = new HashSet<>();

    // getters, setters


@Entity
@JsonInclude(Include.NON_NULL)
public class Department {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    @ManyToMany(mappedBy = "departments", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JsonIgnoreProperties("departments")
    private List<Exam> exams;

   // getters, setters

那个么，这就是我如何发送id的表单给邮递员并导出它们之间的关系的方式吗

我应该做些什么来消除只具有给定id的持久数据之间的关系

@PostMapping("/api/exam")
    public ResponseEntity<?> addExamDepartment(@RequestBody Exam exam) {
        
        Exam ex = examRepository.save(exam);

        if (ex != null) {
            return ResponseEntity.ok(new ApiResponse(true, "Exam Department relation added.", ex));
        }
        return new ResponseEntity<>(new ApiResponse(false, "Exam Department relation not added.", null), 
        HttpStatus.BAD_REQUEST);
    }

{
    "id": 1,
    "departments" : [
        {
        "id" : 1
    }, {
       "id" : 2
    }]
}