In django, is there a way to directly annotate a q

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.