Django: Get latest N number of records per group-CodePudding

Let's say I have the following Django model:

class Team(models.Model):
    name = models.CharField(max_length=255)
    created_at = models.DateTimeField(auto_now_add=True)

I want to write a query to fetch the latest N number of records per team name.

If N=1, the query is very easy (assuming I'm using postgres because it's the only DB that support distinct(*fields)):

Team.objects.order_by("name", "-created_at").distinct("name")

If N is greater than 1 (let's say 3), then it gets tricky. How can I write this query in Django?

CodePudding user response：

Not sure how you can get duplicate names per team, since you have unique=True. But if you plan to remove that to support non-unique names, you can use subqueries like this:

top_3_per_team_name = Team.objects.filter(
    name=OuterRef("name")
).order_by("-created_at")[:3]

Team.objects.filter(
    id__in=top_3_per_team_name.values("id")
)

Although this can be a bit slow, so make sure you have the indexes setup.

Also to note, ideally this can be solved by using Window..[Django-doc] functions using DenseRank..[Django-doc] but unfortunately the latest django version can't filter on windows:

from django.db.models import F
from django.db.models.expressions import Window
from django.db.models.functions import DenseRank

Team.objects.annotate(
    rank=Window(
        expression=DenseRank(),
        partition_by=[F('name'),],
        order_by=F('created_at').desc()
    ),
).filter(rank__in=[range(1,4)]) # 4 is N   1 if N = 3

With the above you get:

NotSupportedError: Window is disallowed in the filter clause.

But there is a plan to support this on Django 4.2 so theoretically the above should work once that is released.

CodePudding user response：

I'm assuming you'll be getting your N from a get request or something, but as long as you have a number you can try limiting your queryset:

Team.objects.order_by("name", "-created_at").distinct("name")[:3] # for N = 3