Django:如何为每个订单创建自定义参考代码？

from django.db import models
from django.db.models.signals import pre_save
from django.dispatch import receiver
import random
import string

class Order(models.Model):
    order_number = models.CharField(max_length=20)
    # 其他订单属性...

@receiver(pre_save, sender=Order)
def generate_reference_code(sender, instance, **kwargs):
    if not instance.order_number:
        reference_code = generate_random_code()
        instance.order_number = reference_code

def generate_random_code():
    length = 8
    letters_and_digits = string.ascii_uppercase + string.digits
    return ''.join(random.choice(letters_and_digits) for _ in range(length))