Let’s play ‘Spot the bug’. We’re building a simple system that shows photos. Each photo has a publish_date and we should only show photos that have been published (i.e. their publish_date is in the past).

class PhotoManager(models.Manager):
    def live(self, as_of=None):
        if as_of is None:
            as_of = timezone.now()
        return super().get_query_set().filter(publish_date__lte=as_of)

And the view to show those photos:

class ShowPhotosView(ListView):
    queryset = Hero.objects.live()

Can you spot the bug? I sure didn’t… until the client complained that newly published photos never showed up on the site. Restarting the server fixed the problem temporarily. The newly published photos would show up, but then any photos published after the server restart again failed to display.

The problem is that the ShowPhotosView class is instantiated when the server starts. ShowPhotosView.queryset gets set to the value returned by Hero.objects.live(). That, in turn, is a QuerySet, but it’s a QuerySet with as_of set to timezone.now() WHEN THE SERVER STARTS UP. That as_of value never gets updated, so newer photos never get captured in the query.

There’s probably multiple ways to fix this, but an easy one is:

class ShowPhotosView(ListView):
    def get_queryset(self):
        return Hero.objects.live()

Now, instead of the queryset being instantiated at server start-up, it’s instantiated only when ShowPhotosView.get_queryset() is called, which is when a request is made.