Django有俩个 app
----------在 Course 的models.py中：
from Shopping.models import Coupon, OrderDetail
class Course:
order_details = GenericRelation(to=OrderDetail)
coupon = GenericRelation(to=Coupon)
-------------在Shopping的models.py中：
from Course.models import Account
class Coupon:
account = models.ForeignKey(to=Account, verbose_name="拥有者", on_delete=None, related_name="coupons")
class OrderDetail:
pass
报错：ImportError: cannot import name 'xxx'
原因：暂不清楚
解决方案：使用app_name.class_name的方式，注意不是app_name.models.class_name
在 Course 的models.py中：
class Course:
order_details = GenericRelation(to="Shopping.OrderDetail")
coupon = GenericRelation(to="Shopping.Coupon")
在Shopping的models.py中：
class Coupon:
account = models.ForeignKey(to="Course.Account", verbose_name="拥有者", on_delete=None, related_name="coupons")
class OrderDetail:
pass