How to aggregate over a single queryset in Django?

Question

Short description: given a queryset myQueryset, how do I select max("myfield") without actually retrieving all rows and doing max in python?

The best I can think of is max([r["myfield"] for r in myQueryset.values("myfield")]), which isn't very good if there are millions of rows.

Long description: Say I have two models in my Django app, City and Country. City has a foreign key field to Country:

class Country(models.Model):
    name = models.CharField(max_length = 256)

class City(models.Model):
    name = models.CharField(max_length = 256)
    population = models.IntegerField()
    country = models.ForeignKey(Country, related_name = 'cities')

This means that a Country instance has .cities available. Let's say I now want to write a method for Country called highest_city_population that returns the population of the largest city. Coming from a LINQ background, my natural instinct is to try myCountry.cities.max('population') or something like that, but this isn't possible.

Answer 1

Use Aggregation (new in Django 1.1). You use it like this:

>>> from django.db.models import Max
>>> City.objects.all().aggregate(Max('population'))
{'population__max': 28025000}

To get the highest population of a City for each Country, I think you could do something like this:

>>> from django.db.models import Max
>>> Country.objects.annotate(highest_city_population = Max('city__population'))