AttributeError：“ NoneType”对象没有属性，因为相关的对象没有在视图中

Question

我的应用程序（部署到Heroku fwiw）有两个模型对象， Org和Event具有相应的对象视图，没有问题/错误。 当我引入第三个对象Theme并修改了一些URL路由以使用其Slug时，我现在得到以下err请求Org的视图（其URL路径将org.slug作为参数的视图）：

AttributeError: 'NoneType' object has no attribute 'org'和404

orgs.models.py

class Org(models.Model):
     name = models.CharField(max_length=100, unique=True)
     slug = AutoSlugField(populate_from='name')
     themes = models.ManyToManyField('common.Theme', related_name='orgs', blank=True)
     ...

     def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('org-detail', args=[self.slug])

class Event(models.Model):
      org = models.ForeignKey('Org', on_delete=models.CASCADE, related_name='events')
      name = models.CharField(max_length=120)
      slug = AutoSlugField(populate_from='name', unique_with='org', sep='-')
      ...

      def __str__(self):
        return self.name

      def get_absolute_url(self):
        return reverse('event-detail', args=[self.org.slug, self.slug])

common.models.py

class Theme(models.Model):
    name = models.CharField(max_length=50)
    slug = AutoSlugField(populate_from='name')

    class Meta:
        ordering = ['name']

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('themes', args=[self.slug])

orgs.urls.py

urlpatterns = [
    url(r'^org/(?P<org_slug>[-\w]+)/?$', views.org, name='org-detail'),
    url(r'^org/(?P<org_slug>[-\w]+)/event/(?P<slug>[-\w]+)/', include([
        url(r'^$', views.event, name='event-detail'),
        ...
    ])),
    ...
]

common.urls.py

urlpatterns = [
     url(r'^(?P<slug>[-\w]+)/?$', views.theme, name='theme'),
     ...
]

orgs.views.py

def event(request, org_slug, slug):

     event = Event.objects.filter(slug=slug).order_by('id').first()

     if event.org.slug != org_slug:
         raise Http404

     event_url = '{}{}{}'.format(settings.DEFAULT_PROTOCOL, settings.APP_DOMAIN, event.get_absolute_url())

     context = {
         'event': event,
         'event_url': event_url,
         'event_url_encode': urlquote_plus(event_url),
     }

     return render(request, 'orgs/event.html', context)


def org(request, slug, role=None):
    try:
        org = Org.objects.get(slug=slug)
    except Org.DoesNotExist:
        return HttpResponseBadRequest()

    context = {
        'org': org,
    }

    return render(request, 'orgs/org.html', context)

views.event返回预期的有效响应。 views.org曾经使用过，但是现在返回404。日志err实际上显示了views.event生成的err，但是我不明白为什么：

    Traceback (most recent call last): 
    File "/app/.heroku/python/lib/python3.6/site-packages/django/core/handlers/exception.py", line 41, in inner 
    response = get_response(request) 
    File "/app/.heroku/python/lib/python3.6/site-packages/django/core/handlers/base.py", line 249, in _legacy_get_response 
    response = self._get_response(request) 
    File "/app/.heroku/python/lib/python3.6/site-packages/django/core/handlers/base.py", line 187, in _get_response 
    response = self.process_exception_by_middleware(e, request) 
File "/app/.heroku/python/lib/python3.6/site-packages/channels/handler.py", line 237, in process_exception_by_middleware 
return super(AsgiHandler, self).process_exception_by_middleware(exception, request) 
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/handlers/base.py", line 185, in _get_response 
    response = wrapped_callback(request, *callback_args, **callback_kwargs) 
    File "/app/.heroku/python/lib/python3.6/contextlib.py", line 52, in inner 
    return func(*args, **kwds) 
    File "/app/project/utils/helpers.py", line 28, in _decorated 
    return view(request, *args, **kwargs) 
    File "/app/.heroku/python/lib/python3.6/site-packages/htmlmin/decorators.py", line 13, in minify 
    response = f(*args, **kwargs) 
    File "/app/orgs/views.py", line 243, in event 
    if event.org.slug != org_slug: 
    AttributeError: 'NoneType' object has no attribute 'org' 

在调试模式下，Django说： No Theme matches the given query. 由project.utils.helpers._decorated为GET app.com/org/valid-org-slug 。 为什么在该查询中调用主题（或将主题的子弹用作arg而不是组织？）-views.event或views.org未使用或未请求它？ 如果view.event返回正确的对象，则如何将NoneType Event设置为NoneType？ 谢谢

Answer 1

实际上，代码中的问题在于first（） 。 如果查询集中没有匹配的对象，则first返回None。 您应该像这样更改行：

events = Event.objects.filter(slug=slug).order_by('id')
if events.exists():
    event = events.first()

Answer 2

我通过更改common.urls中的url路径消除了错误

url(r'^(?P<slug>[-\w]+)/?$', views.theme, name='theme'),

至：

url(r'^foo/(?P<slug>[-\w]+)/?$', views.theme, name='theme'),

之后，views.event和views.org会按预期工作。 我使用的url配置引起了正则表达式问题，该问题创建了基础的NoneType错误。 我仍然不完全了解正则表达式问题的原因/来源。