Django annotate with model values - sql

Question

The Django docs on aggregation give the following example for annotations:
for store in Store.objects.all():
store.min_price # Error! min_price not defined!
for store in Store.objects.annotate(min_price=Min('books__price')):
store.min_price # Fine
However, we only annotated a single field. We only know what the price of the cheapest book is, but not exactly which is the cheapest book. What if I wanted the result of the annotation be precisely that book, not just its price? (I'll call this function or class AggregateRelation)
for store in Store.objects.annotate(
cheapest_book=AggregateRelation('books__price', Min)
):
store.cheapest_book.price
store.cheapest_book.title
Is there a way to do this?
I checked up FilteredRelation but that's only useful for filtering. It does not truly retrieve the instances.

Answer 1

If (and only if) you are using PostgreSQL, then I think there's an answer using .distinct(fields)
Book.objects.order_by('store_id', 'price').distinct('store_id')
should get you the cheapest book for each store if I've understood the documentation (linked above) right. Adding select_related() should mean that you don't hit the DB again when you refer to book.store.whatever
If you don't have PostgreSQL I think you can use a SubQuery. The following is my attempt to follow the documentation:
from django.db.models import OuterRef, Subquery
cheapest = Book.objects.filter(store=OuterRef('pk')).order_by('price')
stores = Store.objects.annotate(cheapest_book_pk=Subquery(cheapest.values('pk')[:1]))
Something like this will get you a list of Store objects, each annotated with the pk of the cheapest book that it is selling.
Please tell me whether I've got this right!