Django: output aggregation of aggregation ordered by counts

Question

I'm trying to output the following data in my django templates.

Countries would be ordered descending by # of stories. Cities would be ordered descending by # of stories (under that country)

Country A(# of stories)
  City A (# of stories)
  City B (# of stories)

Country B(# of stories)
  City A (# of stories)
  City B (# of stories)

My models are the following:

# Create your models here.
class Country(models.Model):
    name = models.CharField(max_length=50)

class City(models.Model):
    name = models.CharField(max_length=50)
    country = models.ForeignKey(Country)

class Story(models.Model):
    city = models.ForeignKey(City)
    country = models.ForeignKey(Country)
    name = models.CharField(max_length=255)

What's the easiest way to do this?

Answer 1

http://docs.djangoproject.com/en/dev/ref/templates/builtins/?from=olddocs#regroup

Answer 2

This solution worked for me. You will need to tweak it to pass it to a template though.

from django.db.models import Count
all_countries = Country.objects.annotate(Count('story')).order_by('-story__count')

for country in all_countries:
    print "Country %s (%s)" % (country.name, country.story__count)
    all_cities = City.objects.filter(country = country).annotate(Count('story')).order_by('-story__count')
    for city in all_cities:
        print "\tCity %s (%s)" % (city.name, city.story__count)

Update

Here is one way of sending this information to the template. This one involves the use of a custom filter.

@register.filter
def get_cities_and_counts(country):
    all_cities = City.objects.filter(country = country).annotate(Count('story')).order_by('-story__count')
    return all_cities

View:

def story_counts(request, *args, **kwargs):
    all_countries = Country.objects.annotate(Count('story')).order_by('-story__count')
    context = dict(all_countries = all_countries)
    return render_to_response(..., context)

And in your template:

{% for country in all_countries %}
    <h3>{{ country.name }} ({{ country.story__count }})</h3>
    {% for city in country|get_cities_and_counts %}
        <p>{{ city.name }} ({{ city.story__count }})</p>
    {% endfor %}
{% endfor %}

Update 2

Variant with a custom method in model.

class Country(models.Model):
    name = models.CharField(max_length=50)

    def _get_cities_and_story_counts(self):
        retrun City.objects.filter(country = self).annotate(Count('story')).order_by('-story__count')
    city_story_counts = property(_get_cities_and_story_counts)

This lets you avoid defining a filter. The template code changes to:

{% for country in all_countries %}
    <h3>{{ country.name }} ({{ country.story__count }})</h3>
    {% for city in country.city_story_counts %}
        <p>{{ city.name }} ({{ city.story__count }})</p>
    {% endfor %}
{% endfor %}