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

问题

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?

回答1:

No, it's not possible to combine them into a single query.

You can read the following blog post to find two workarounds.

回答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.

来源：https://stackoverflow.com/questions/12135174/in-django-is-there-a-way-to-directly-annotate-a-query-with-a-related-object-in