Use get_queryset() method or set queryset variable?

Question

These two pieces of code are identical at the first blush:

class IndexView(generic.ListView):     template_name = 'polls/index.html'     context_object_name = 'latest_poll_list'     queryset = Poll.active.order_by('-pub_date')[:5] 

and

class IndexView(generic.ListView):     template_name = 'polls/index.html'     context_object_name = 'latest_poll_list'      def get_queryset(self):         return Poll.active.order_by('-pub_date')[:5] 

Is there any difference between them? And if it is:

What approach is better? Or when setting queryset variable is better than override the get_queryset method? And vice versa.

Answer 1

In your example, overriding queryset and get_queryset have the same effect. I would slightly favour setting queryset because it's less verbose.

When you set queryset, the queryset is created only once, when you start your server. On the other hand, the get_queryset method is called for every request.

That means that get_queryset is useful if you want to adjust the query dynamically. For example, you could return objects that belong to the current user:

class IndexView(generic.ListView):     def get_queryset(self):         """Returns Polls that belong to the current user"""         return Poll.active.filter(user=self.request.user).order_by('-pub_date')[:5] 

Another example where get_queryset is useful is when you want to filter based on a callable, for example, return today's polls:

class IndexView(generic.ListView):     def get_queryset(self):         """Returns Polls that were created today"""         return Poll.active.filter(pub_date=date.today()) 

If you tried to do the same thing by setting queryset, then date.today() would only be called once, when the view was loaded, and the view would display incorrect results after a while.

class IndexView(generic.ListView):     # don't do this!     queryset = Poll.active.filter(pub_date=date.today()) 

Answer 2

Other answers have missed an important implication of the fact that the queryset attribute is evaluated when the process starts. Because you aren't just creating a queryset, you're actually slicing it, the query will be evaluated at that point. That means that you will only ever get the top 5 polls at that moment, and they won't refresh even if you create another one, until the process is restarted.

This is exactly when you should be using get_queryset().