在Django中间件中导入模型时遭遇App_label相关报错求助

你遇到的这个错误本质上是中间件加载时机早于Django应用初始化完成的时机——Django在启动时会先加载中间件，此时notifications应用的模型还没完成注册，所以直接导入模型或者用错误的AppConfig调用方式都会触发报错。下面给你几个可行的解决方案：

方案1：延迟导入模型到process_request内部 #

把模型的导入逻辑从模块顶部移到process_request方法内部，这样只有当请求实际进来时才会加载模型，此时所有应用已经完成初始化：

class CheckNotificationMiddleware(object):
    """ Checks clicked notification to mark it as read """
    def process_request(self, request):
        if request.user.is_authenticated():
            # 延迟导入，此时应用已完成初始化
            from notifications.models import Notification
            from django.core.exceptions import ObjectDoesNotExist
            
            notification_id = request.GET.get('notification_id')
            if notification_id:
                try:
                    notification = Notification.objects.get(
                        pk=notification_id, 
                        status=0, 
                        recipient=request.user
                    )
                    notification.status = 1
                    notification.save()
                except ObjectDoesNotExist:
                    pass

方案2：正确使用AppConfig获取模型 #

你之前调用AppConfig.get_model('Notification')的方式有误，需要先获取对应应用的AppConfig实例，再调用get_model方法。推荐用apps.get_model的快捷方式：

from django.apps import apps
from django.core.exceptions import ObjectDoesNotExist

class CheckNotificationMiddleware(object):
    """ Checks clicked notification to mark it as read """
    def process_request(self, request):
        if request.user.is_authenticated():
            notification_id = request.GET.get('notification_id')
            if notification_id:
                # 指定应用标签和模型名获取模型
                Notification = apps.get_model('notifications', 'Notification')
                try:
                    notification = Notification.objects.get(
                        pk=notification_id, 
                        status=0, 
                        recipient=request.user
                    )
                    notification.status = 1
                    notification.save()
                except ObjectDoesNotExist:
                    pass

也可以显式获取应用的AppConfig实例：

from django.apps import AppConfig
from django.core.exceptions import ObjectDoesNotExist

class CheckNotificationMiddleware(object):
    """ Checks clicked notification to mark it as read """
    def process_request(self, request):
        if request.user.is_authenticated():
            notification_id = request.GET.get('notification_id')
            if notification_id:
                # 先获取notifications应用的配置实例
                notifications_app = AppConfig.get_app_config('notifications')
                Notification = notifications_app.get_model('Notification')
                try:
                    notification = Notification.objects.get(
                        pk=notification_id, 
                        status=0, 
                        recipient=request.user
                    )
                    notification.status = 1
                    notification.save()
                except ObjectDoesNotExist:
                    pass

额外检查项 #

• 确认notifications应用的models.py中，Notification模型定义正常：如果是自定义应用，无需额外声明app_label（只要应用在INSTALLED_APPS中即可）；如果是第三方应用，确保已正确安装（比如通过pip安装的版本适配Django 1.11）
• 执行python manage.py clearcache清除Django缓存，避免旧缓存导致的应用初始化异常

内容的提问来源于stack exchange，提问作者Stephen Nyamweya