Aggregate or Annotaion on previously Annotated values

[eye-see-you@…]

from django.db.models import Count, Sum, F

myModel.objects \

.annotate(a_count=Count('field_a'), b_count=Count('field_b')) \
.annotate(total=F('a_count')+F('b_count')) \
.order_by('total')

This query is going to raise AttributeError: 'ExpressionNode' object has no attribute 'split'

I don't find any method in django ORM to support such kind of operations (in which two fields are involved).