Django - How to access filtered queryset in the dispatch method?

Question

I have the following code:

class MyViewSet(ModelViewSet):
    ...
    filter_backends = (...)

    def dispatch(self, request, *args, **kwargs):
        response = super(MyViewSet, self).dispatch(
            request,
            *args,
            **kwargs
        )
        ... # do something with the response
        return response

Inside the dispatch method, I can retrieve the filtered data with response.data , so I assume the custom filter backend is working properly. However, I also want to do something with the queryset as well (eg call count() ), after the filter has been applied to it. The problem is that self.queryset and self.get_queryset() return the whole, non-filtered queryset.

So how do get the version of queryset to which the filter has been applied in the dispatch method, if it's possible at all?

Answer 1

You could use the filter_queryset method. It will filter it with whichever filter backend is in use. See its definition in GenericAPIView class.

def filter_queryset(self, queryset):
    """
    Given a queryset, filter it with whichever filter backend is in use.
    You are unlikely to want to override this method, although you may need
    to call it either from a list view, or from a custom `get_object`
    method if you want to apply the configured filtering backend to the
    default queryset.
    """
    for backend in list(self.filter_backends):
        queryset = backend().filter_queryset(self.request, queryset, self)
    return queryset

So you can filter the queryset like,

filtered_queryset = self.filter_queryset(self.get_queryset())
# Then do something with the filtered queryset

Note that self.filter_queryset(self.get_queryset()) will filter the queryset again even if the queryset was already filtered during the following super call

super(MyViewSet, self).filter_queryset(
                                self.queryset()
                                ).

Following is one way to overcome the duplicate filter call. You could override the filter_queryset method and attach the filtered queryset to the instance.( please note that this solution is not tested )

ie, something like this.

class MyViewSet(ModelViewSet):
    ...
    filter_backends = (...)

    def dispatch(self, request, *args, **kwargs):
        response = super(MyViewSet, self).dispatch(
            request,
            *args,
            **kwargs
        )
        ... # do something with the response
        # After this super call `_filtered_query_set` attribute will be set.
        # so use `self._filtered_query_set` wherever needed
        return response

   def filter_queryset(self, queryset):
       filtered_query_set = super(MyViewSet, self).filter_queryset(
                            self.queryset()
                            )
       self._filtered_query_set = filtered_query_set
       return filtered_query_set