'Cannot assign "%r": the current database router prevents this relation.' % value)

https://code.djangoproject.com/ticket/20337

#20337 (ValueError when database router prevents relation is unclear) – Django

In my router, the allow_relation method was set incorrectly, and as a result, two models from separate apps weren't allowed to reference each other. The ValueError I received looked like this:

routers.py에 아래 내용 추가

    def allow_relation(self, obj1, obj2, **hints) :

        if obj1._meta.app_label == 'auth' or obj2._meta.app_label == 'auth' :

            return True 

        return None