spring JPA)如何在只提供外键和考虑服务的情况下保存实体？

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    // other fields...
}

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

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "user_id", nullable = false)
    private User user;

    // other fields...
}

@Service
public class OrderService {

    @PersistenceContext
    private EntityManager entityManager;

    @Transactional
    public void saveOrderWithUserId(Order order, Long userId) {
        User user = entityManager.find(User.class, userId);
        if (user != null) {
            order.setUser(user);
            entityManager.persist(order);
        } else {
            throw new IllegalArgumentException("User not found with id: " + userId);
        }
    }
}

@RestController
@RequestMapping("/orders")
public class OrderController {

    @Autowired
    private OrderService orderService;

    @PostMapping
    public ResponseEntity<?> createOrder(@RequestBody Order order, @RequestParam Long userId) {
        try {
            orderService.saveOrderWithUserId(order, userId);
            return ResponseEntity.ok().build();
        } catch (IllegalArgumentException e) {
            return ResponseEntity.badRequest().body(e.getMessage());
        }
    }
}