这是我第一次使用Spring框架，在这里我试图创建两个表类和学生，具有多对一关系，我无法创建新的学生，我尝试，我得到一个错误的请求错误400，thsi是我的学生实体

@Entity
@Data
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private Long id;
    private String codeM;
    private String firstName;
    private String lastName;
    @ManyToOne
    private Classes classes;
}

这是学生控制器

@RestController
@RequestMapping("/student")
@AllArgsConstructor
@Slf4j
public class StudentController {
    final StudentService studentService;

    // create new student
    @PostMapping("/")
    public ResponseEntity<?> create(@RequestBody(required = false) Student student) throws Exception {
        if(student == null)
            return ResponseEntity.status(HttpStatus.NOT_FOUND).body("Student is null");
        Student savedStudent = studentService.create(student);
        return ResponseEntity.status(HttpStatus.CREATED).body(savedStudent);
    }

    @GetMapping("/{CodeM}")
    public Student findByCodeM(@PathVariable String CodeM) throws Exception{
        return studentService.findByCodeM(CodeM);
    }

    @PutMapping("/{CodeM}")
    public ResponseEntity<?> update(@PathVariable String CodeM, @RequestBody Student student) throws Exception{
        if(student == null){
            return ResponseEntity.status(HttpStatus.NOT_FOUND).body("Student is null");
        }
        Student savedStudent = studentService.update(student);
        return ResponseEntity.status(HttpStatus.OK).body(savedStudent);
    }

    @GetMapping("/")
    public List<Student> findAll() {
        return studentService.findAll();
    }
}

get请求工作，但POST和Put请求不工作