Problems with dictsort and *_set.count in template

Using Django Using the ORM
1

Context

I have these two models:

class Country(models.Model):                                                           
  name = models.CharField(max_length=128, unique=True, validators=[validators.labelvalue])

class City(models.Model):
  name = models.CharField(max_length=128, unique=True, validators=[validators.labelvalue])
  country = models.ForeignKey('myapp.Country', on_delete=models.CASCADE)

If I’m not mistaken, according to the docs country.city_set would automatically exist, and country.city_set.count() will return the number of cities belonging to that country.

related_name is not used in any of those models.

My problem

Now, in a template, I’m trying to loop through the countries, and I would like them sorted by number of cities. So, this is what I do (simplified):

views.py

from django.views.generic import ListView

class CountryList(ListView):
  queryset = models.Country.objects.prefetch_related(
    "city_set",
  )

template.html

{% for country in country_list|dictsort:"city_set.count" %}

However, is not working and just renders a blank page. If I replace city_set.count with name, it works fine, so I guess the problem must be using city_set.count.

What could be issue? Thanks in advance.

2

From the docs for dictsort:

Takes a list of dictionaries and returns that list sorted by the key given in the argument.

city_set.count is not an attribute in the Country object. The key would need to be city_set. (count would be an attribute in the city_set object.)

You could add an annotation to your query to add the count of city_set as an attribute of the elements in the queryset, and reference it in your dictsort filter.

3

You could add an annotation to your query to add the count of city_set as an attribute of the elements in the queryset

I have modified the query in this way:

class CountryList(ListView):
  queryset = models.Country.objects.prefetch_related(
    "city_set",
  ).annotate(Count("city_set"))

But now I get the following error:

django.core.exceptions.FieldError: Cannot resolve keyword 'city_set' into field.
Choices are: id, name, city
4

I could not annotate a count on the city_set field. Instead, I needed to use a city field.
This fixed it:

views.py

class CountryList(ListView):
  queryset = models.Country.objects.prefetch_related(
    "city_set",
  ).annotate(num_cities=Count("city"))

template.html

{% for country in country_list|dictsort:"num_cities" %}

Thanks @KenWhitesell.