In django, is there a way to directly annotate a query with a related object in single query?

Question

Consider this query:

query = Novel.objects.< ...some filtering... >.annotate(
    latest_chapter_id=Max("volume__chapter__id")
)

Actually what I need is to annotate each Novel with its latest Chapter object, so after this query, I have to execute another query to select actual objects by annotated IDs. IMO this is ugly. Is there a way to combine them into a single query?

Answer 1

Yes, using Subqueries, docs: https://docs.djangoproject.com/en/3.0/ref/models/expressions/#subquery-expressions

latest_chapters = Chapter.objects.filter(novel = OuterRef("pk"))\
    .order_by("chapter_order")

novels_with_chapter = Novel.objects.annotate(
    latest_chapter = Subquery(latest_chapters.values("chapter")[:1]))

Tested on Django 3.0

The subquery creates a select statement inside the select statement for the novels, then adds this as an annotation. This means you only hit the database once.

I also prefer this to Rune's answer as it actually annotates a Novel object.

Hope this helps, anyone who came looking like much later like I did.

Answer 2

Yes, it's possible.

To get a queryset containing all Chapters which are the last in their Novels, simply do:

from django.db.models.expressions import F
from django.db.models.aggregates import Max

Chapters.objects.annotate(last_chapter_pk=Max('novel__chapter__pk')
    ).filter(pk=F('last_chapter_pk'))

Tested on Django 1.7.