class ProductFilter(FilterSet):
    class Meta:
        model = Product
        fields = ('brand', )

    brand = ModelMultipleChoiceFilter(queryset=Brand.objects.all(),
                                      widget=BrandFilterWidget('checkbox'),
                                      to_field_name='id')
    def get_brands_queryset(self):
        return Brand.objects.filter(products__in=self.queryset).distinct('id')