ViewSet class variable

Question

Now I have the following logic implemented for a GET request in Django Rest Framework:

class SomeViewSet(mixins.ListModelMixin,
                  GenericViewSet):
    count = None

    def get_queryset(self):
        query_set = ... # some_logic
        self.count = query_set.count()
        return query_set

    def list(self, request, *args, **kwargs):
        response = super().list(request, *args, **kwargs)
        response.data = {'count': self.count,
                         'data': response.data}
        return response

That is, the queryset is calculated according to complex logic, and it may contain a different number of objects that need to be returned in a GET request, since I don’t have access to the query_set variable inside the list function and I don’t want to copy the query_set calculation logic, I decided do it with a class variable.

But still, the feeling that this is not very correct does not leave. What other options are there?

What should be returned in case you paginate? The number of items on the "page" or the total number of elements? — Willem Van Onsem, Feb 08 '23 at 14:39
@WillemVanOnsem total number of elements in the `query_set` variable, regardless of pages — Anonymous Wizard, Feb 08 '23 at 15:02

Sunderam Dubey · Accepted Answer · 2023-02-08T15:39:03.873

You can use self.get_queryset() inside the list method instead of using a class variable. The get_queryset method will be executed every time you call it, and it will return the current queryset so:

class SomeViewSet(mixins.ListModelMixin,
                  GenericViewSet):

    def get_queryset(self):
        return ... # some_logic

    def list(self, request, *args, **kwargs):
        queryset = self.get_queryset()
        response = super().list(request, *args, **kwargs)
        response.data = {'count': queryset.count(),
                         'data': response.data}
        return response

Edit:

To avoid the issue of multiple database queries, you can make use of the queryset that is already retrieved by the ListModelMixin and stored in the response.data attribute so:

class SomeViewSet(mixins.ListModelMixin,
                  GenericViewSet):

    def get_queryset(self):
        return ... # some_logic

    def list(self, request, *args, **kwargs):
        response = super().list(request, *args, **kwargs)
        queryset = response.data
        response.data = {'count': len(queryset),
                         'data': queryset}
        return response

Thanks for the answer. Wouldn't this be a double query to the database, because the `list` method in the `ListModelMixin` also calls the `get_query_set` function? — Anonymous Wizard, Feb 08 '23 at 15:14
@Анонимныйволшебник Yes, calling `self.get_queryset()` in the list method and in the ListModelMixin could result in multiple database queries. — Sunderam Dubey, Feb 08 '23 at 15:37
Sorry, can i ask you? Any suggestion if i return slice in `get_query_set` but need to get the full length (in `list`) before i slice? If you help i can upvote your other answer — Anonymous Wizard, Feb 11 '23 at 10:43

ViewSet class variable

1 Answers1

Edit: