Django多数据库路由器中的allow_relation

Question

我的通用DatabaseAppsRouter类：

class DatabaseAppsRouter(object):
    def db_for_read(self, model, **hints):
        if settings.APPS_DATABASE_MAPPING.has_key(model._meta.app_label):
            return settings.APPS_DATABASE_MAPPING[model._meta.app_label]
        return DEFAULT_DB_ALIAS

    def db_for_write(self, model, **hints):
        if settings.APPS_DATABASE_MAPPING.has_key(model._meta.app_label):
            return settings.APPS_DATABASE_MAPPING[model._meta.app_label]
        return DEFAULT_DB_ALIAS

    def allow_relation(self, obj1, obj2, **hints):
        if obj1._meta.app_label == settings.APP_QUES or obj2._meta.app_label == settings.APP_QUES:
           return True
        return None

    def allow_migrate(self, db, model):
        if db == DEFAULT_DB_ALIAS:
            if not(model._meta.app_label in settings.APPS_DATABASE_MAPPING.keys()):
                return True
            else:
                return False
        else:
            return settings.APPS_DATABASE_MAPPING.get(model._meta.app_label) == db
        return None

settings.py：

DATABASE_ROUTERS = ['rest_apis.database_app_router.DatabaseAppsRouter']
APPS_DATABASE_MAPPING = {APP_QUES: 'questions', 
                          'auth':'default'}

我的疑问是，我可以看到控件进入allow_migrate和db_for_write，但是当我正在运行allow_relation时，它不会进入./manage.py migrate

这是正常行为还是我的实施中出现了问题？