SpringBoot 2.1.4.RELEASE应用程序中的SQL错误: 23503，SQLState: 23503

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

    private String name;

    // getters and setters
}

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

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

    // getters and setters
}

@Service
public class OrderService {

    @Autowired
    private OrderRepository orderRepository;

    @Autowired
    private UserRepository userRepository;

    public void createOrder(Long userId) {
        User user = userRepository.findById(userId).orElseThrow(() -> new RuntimeException("User not found"));
        Order order = new Order();
        order.setUser(user);
        orderRepository.save(order);
    }
}