[Django]-Django Query That Get Most Recent Objects From Different Categories

Starting from Django 1.11 and thanks to Subquery and OuterRef, we can finally build a latest-per-group query using the ORM.

hottest_cakes = Cake.objects.filter(
    baked_at=Subquery(
        (Cake.objects
            .filter(bakery=OuterRef('bakery'))
            .values('bakery')
            .annotate(last_bake=Max('baked_at'))
            .values('last_bake')[:1]
        )
    )
)

#BONUS, we can now use this for prefetch_related()
bakeries = Bakery.objects.all().prefetch_related(
    Prefetch('cake_set',
        queryset=hottest_cakes,
        to_attr='hottest_cakes'
    )
)

#usage
for bakery in bakeries:
    print 'Bakery %s has %s hottest_cakes' % (bakery, len(bakery.hottest_cakes))

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.

This should do the job:

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

I was fighting with similar problem and finally come to following solution. It does not rely on order_by and distinct so can be sorted as desired on db-side and also can be used as nested query for filtering. I also believe this implementation is db engine independent, because it’s based on standard sql HAVING clause. The only drawback is that it will return multiple hottest cakes per bakery, if they are baked in that bakery at exactly same time.

from django.db.models import Max, F

Cake.objects.annotate(
    # annotate with MAX "baked_at" over all cakes in bakery
    latest_baketime_in_bakery=Max('bakery__cake_set__baked_at')
    # compare this cake "baked_at" with annotated latest in bakery
).filter(latest_baketime_in_bakery__eq=F('baked_at'))

Cake.objects.filter(bakery__town="Anytown").order_by("-created_at")[:1]

I haven’t built out the models on my end, but in theory this should work. Broken down:

• Cake.objects.filter(bakery__town="Anytown") Should return any cakes whom belong to “Anytown”, assuming the country is not part of the string. The double underscores between bakery and town allow us to access the town property of bakery.
• .order_by("-created_at") will order the results by their created date, most recent first (take note of the - (minus) sign in "-created_at". Without the minus sign, they’d be ordered by oldest to most recent.
• [:1] on the end will return only the 1st item in the list which is returned (which would be a list of cakes from Anytown, sorted by most recent first).

Note: This answer is for Django 1.11.
This answer modified from Queries shown here in Django 1.11 Docs.

@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