如何在通过Django Admin添加新产品时为MySQL生成随机的product_id int

from django.db import models

class Product(models.Model):
    product_id = models.IntegerField(unique=True)
    # 其他字段...

from django.contrib import admin
from .models import Product
import random

class ProductAdmin(admin.ModelAdmin):
    def save_model(self, request, obj, form, change):
        if not obj.product_id:
            # 生成一个随机的product_id
            obj.product_id = random.randint(1000, 9999)
        super().save_model(request, obj, form, change)

admin.site.register(Product, ProductAdmin)

python manage.py makemigrations
python manage.py migrate