Django Query That Get Most Recent Objects From Different Categories

user_183304 0
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I have two models A and B. All B objects have a foreign key to an A object. Given a set of A objects, is there anyway to use the ORM to get a set of B objects containing the most recent object created for each A object.

Here's an simplified example:

class Bakery(models.Model):
    town = models.CharField(max_length=255)

class Cake(models.Model):
    bakery = models.ForeignKey(Bakery, on_delete=models.CASCADE)
    baked_at = models.DateTimeField()

So I'm looking for a query that returns the most recent cake baked in each bakery in Anytown, USA.

greatest-n-per-group django-queryset django python
7 Answers

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.

user_1761860

If you happen to be using PostGreSQL, you can use Django's interface to DISTINCT ON:

recent_cakes = Cake.objects.order_by('bakery__id', '-baked_at').distinct('bakery__id')

As the docs say, you must order by the same fields that you distinct on. As Simon pointed out below, if you want to do additional sorting, you'll have to do it in Python-space.

user_13093320

This should do the job:

from django.db.models import Max
Bakery.objects.annotate(Max('cake__baked_at'))
user_1043490

@Tomasz Zieliński solution above did solve your problem but it did not solve mine, because I still need to filter the Cake. So here is my solution

from django.db.models import Q, Max

hottest_yellow_round_cake = Max('cake__baked_at', filter=Q(cake__color='yellow', cake__shape='round'))

bakeries = Bakery.objects.filter(town='Chicago').annotate(
    hottest_cake_baked_at=hottest_yellow_round_cake
)

hottest_cakes = Cake.objects.filter(
    baked_at__in=[b.hottest_cake_baked_at for b in bakeries]
)

With this approach, you can also implement other things like Filter, Ordering, Pagination for Cakes

user_70118240
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