Django Query That Get Most Recent Objects from Different Categories

Django Query That Get Most Recent Objects From Different Categories

As far as I know, there is no one-step way of doing this in Django ORM, but you can split it into two queries:

from django.db.models import Max

bakeries = Bakery.objects.annotate(
    hottest_cake_baked_at=Max('cake__baked_at')
) 
hottest_cakes = Cake.objects.filter(
    baked_at__in=[b.hottest_cake_baked_at for b in bakeries]
)

If id's of cakes are progressing along with bake_at timestamps, you can simplify and disambiguate the above code (in case two cakes arrives at the same time you can get both of them):

from django.db.models import Max

hottest_cake_ids = Bakery.objects.annotate(
    hottest_cake_id=Max('cake__id')
).values_list('hottest_cak‌​e_id', flat=True)

hottest_cakes = Cake.objects.filter(id__in=hottest_cake_ids)

BTW credits for this goes to Daniel Roseman, who once answered similar question of mine:

http://groups.google.pl/group/django-users/browse_thread/thread/3b3cd4cbad478d34/3e4c87f336696054?hl=pl&q=

If the above method is too slow, then I know also second method - you can write custom SQL producing only those Cakes, that are hottest in relevant Bakeries, define it as database VIEW, and then write unmanaged Django model for it. It's also mentioned in the above django-users thread. Direct link to the original concept is here:

http://web.archive.org/web/20130203180037/http://wolfram.kriesing.de/blog/index.php/2007/django-nice-and-critical-article#comment-48425

Hope this helps.

Django: query latest posts from different categories

from django.db.models import Max

categories = Category.objects.annotate(most_recent=Max(post__date)).order_by('-most_recent')[:5]

posts = list()
for category in categories:
  posts.append(category.post_set.latest())

That annotates the categories with the date of the most recent post as the value of most_recent, which you can then order by to get the 5 most recent categories.

Then, just loop through the categories and pull out the latest post for each. Done.

Filtering X most recent entries in each category of queryset

pk_to_rank = queryset.annotate(rank=Window(
            expression=DenseRank(),
            partition_by=('content_type_id',),
            order_by=F('access_time').desc(),
        )).values_list('pk', 'rank', named=True)

        pks_list = sorted(log.pk for log in pk_to_rank if log.rank <= value)

        return queryset.filter(pk__in=pks_list)

Managed to do it only this way by spliting queryset in 2 parts. Option with 3 unions is also possible but what if we have 800 options instead 3 - make 800 unions()??? ges not...

Get the latest record with filter in Django

obj= Model.objects.filter(testfield=12).order_by('-id')[0]

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