django之register_model(self, app_label, model):

前面在阅读apps_install源码时，一直不明白app的model是什么时候导入的，今天在看modelbase源码时，看到了new_class._prepare()

new_class._meta.apps.register_model(new_class._meta.app_label, new_class)也就是说，在生成model类时会注册到自己所属的app中，在导入models.py时，就会生成model类。

def register_model(self, app_label, model):
        # Since this method is called when models are imported, it cannot
        # perform imports because of the risk of import loops. It mustn't
        # call get_app_config().
        model_name = model._meta.model_name
        app_models = self.all_models[app_label]#默认词典
        if model_name in app_models:
            if (model.__name__ == app_models[model_name].__name__ and
                    model.__module__ == app_models[model_name].__module__):
                warnings.warn(
                    "Model '%s.%s' was already registered. "
                    "Reloading models is not advised as it can lead to inconsistencies, "
                    "most notably with related models." % (app_label, model_name),
                    RuntimeWarning, stacklevel=2)
            else:
                raise RuntimeError(
                    "Conflicting '%s' models in application '%s': %s and %s." %
                    (model_name, app_label, app_models[model_name], model))
        app_models[model_name] = model#往self.all_models[app_label]加入了modle
        self.do_pending_operations(model)
        self.clear_cache()

def check_models_ready(self):
        """
        Raises an exception if all models haven't been imported yet.
        """
        if not self.models_ready:
            raise AppRegistryNotReady("Models aren't loaded yet.")

def check_models_ready(self):
        """
        Raises an exception if models haven't been imported yet.
        """
        if self.models is None:#如果该app没有models，就出错。
            raise AppRegistryNotReady(
                "Models for app '%s' haven't been imported yet." % self.label)