Case-When aggregation over aggregated fields doesn't work since 4.2

[Denis Roldán]

This same QuerySet was working on 4.1.X and prior versions and stopped working on 4.2.X:

annotated_users  = users_qs.annotate(
            total_things=Subquery(
                OtherModel.objects.filter(user_id=OuterRef("pk")).annotate(
                    total_objs=F("total")
                ).values("total_objs")
            )
)

annotated_users.aggregate(
            sum_total_objs=Sum("total_things"),
            avg_conversion_rate=Case(
                When(
                    sum_total_objs=0,
                    then=0,
                ),
                default=Round(
                    (Sum("sum_total_confirmed_objs") / Sum("sum_total_objs")) * 100, 2
                ),
                output_field=FloatField(),
            )
)

As you can see sum_total_objs is an aggregated field that is also used on a second field to calculate the conversion rate. To avoid a zero division problem, we were using a Case-When clause over that field. It works well on any 4.1 and prior versions but stopped working since 4.2, raising a FieldError like:

Cannot resolve keyword 'sum_total_objs' into field

Thanks for the support!