Annotating a QuerySet within a FilterSet Class

[MichielDeKoninck]

Hi everyone!

I am trying to use a FilterSet Class in support of a ViewSet to get the view that I need.
For this I would like to:

• Annotate within a method of the FilterSet Class based on some value inputs (here it's a date range)
• Filter on the annotated value to get only specific objects

I want to do this annotation in the FilterSet because it is where I have the inputs (date range) readily available.

Question: Can I annotate on a QuerySet within a FilterSet?

I am unable to do the annotation and get the error below (error 1). FYI: I do not get this error if I make a dummy annotation on the same field directly in the ViewSet. And the field num_week_specific_matches has been declared as an IntegerField() in the serializer_class of the Viewset.

The Reduced Code:

class RequestsFilter(filters.FilterSet):
    """Filter set for requests"""
    only_week_specific = filters.DateFromToRangeFilter(
         field_name="only_week_specific",
         method="get_week_specific_matches")


    @staticmethod
    def get_week_specific_matches(
        queryset: QuerySet,
        name:str, # pylint: disable=unused-argument
        value:str) -> QuerySet:
        """filter on if this requests have recent matches

        Args:
            queryset (QuerySet): incoming queryset
            name (str): field name
            value (str): value that determines whether to toggle the "recent" view for the request

        Returns:
            QuerySet: returns a new queryset
        """

        start_date = value.start
        end_date = value.stop

        creation_date_true = (Q(match__created_at__gte=start_date) &
                                Q(match__created_at__lte=end_date))
        update_date_true= (Q(match__last_updated_at__gte=start_date) &
                            Q(match__last_updated_at__lte=end_date))

        queryset.annotate(
            num_week_specific_matches=Count(
                'match',
                filter=(creation_date_true | update_date_true)
            )
        )

       
    return queryset       
    #return queryset.filter(Q(num_week_specific_matches__gt=0))
       
    class Meta:
        model = Request
        fields = ['only_week_specific']

The Error:

1. Error when purely annotating in FilterSet without filter on annotated value

Got AttributeError when attempting to get a value for fieldnum_week_specific_matcheson serializerAllRequestSerializer. The serializer field might be named incorrectly and not match any attribute or key on the Requestinstance. Original exception text was: 'Request' object has no attribute 'num_week_specific_matches'.

2. Error when trying to filter on the non-existant pseudofield --> is expected because annotation fails

Cannot resolve keyword 'num_week_specific_matches' into field. Choices are: city, country, created_at, customer, geopoint, house_number, id, is_deleted, last_updated_at, match, maximum_gfa, minimum_gfa, minimum_site_area, name, num_matches, num_selected_matches, owner, postal_code, radius, requested_destination, source, street

Will try to post a minimal working version ASAP.

[MichielDeKoninck]

Good news:

explicitly creating a new queryset with annotation and then filtering on that, seems to solve the problem.
Thus just doing:

 f= queryset.annotate(
            num_week_specific_matches=Count(
                'match',
                filter=(creation_date_true | update_date_true)
            )
        )

       
        
    return f.filter(Q(num_week_specific_matches__gt=0))

Solves the problem.
Though I am still not 100% on the why behind this.