Не получается использовать пользовательские запросы ORM, как QuerySet методы.

class CityManager(models.Manager):
    
    def with_dencity(self):
        return self.aggregate(dencity=Sum(F('population') /F('area'),output_field=FloatField()))


class CountryManager(models.Manager):
    
    def with_num_cities(self):
        return self.aggregate(num_cities=Count('cities'))

    def with_biggest_city_size(self):
        return self.aggregate(biggest_city_size=Max('cities__area'))


class Country(models.Model):
    objects = CountryManager()
    name = models.CharField(
        verbose_name='Название',
        max_length=255
    )

    def __str__(self):
        return '{}'.format(self.name)

class City(models.Model):
    objects = CityManager()
    country = models.ForeignKey(
        Country,
        verbose_name='Страна',
        related_name='cities',
        on_delete=models.CASCADE
    )
    name = models.CharField(
        verbose_name='Название',
        max_length=255
    )
    population = models.FloatField(
        verbose_name='Население'
    )
    area = models.FloatField(
        verbose_name='Площадь'
    )

    def __str__(self):
        return '{} {} {}'.format(self.name, self.population, self.area)

>>> c = City.objects.filter(name='Москва').aggregate(dencity=Sum(F('population') /F('area'),output_field=FloatField()))
>>> c
{'dencity': 10.0}
>>> c = City.objects.filter(name='Москва').with_dencity()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'QuerySet' object has no attribute 'with_dencity'
>>>
>>>
>>>
>>> c = Country.objects.filter(name='США').aggregate(num_cities=Count('cities'), biggest_city_size=Max('cities__area'))
>>> c
{'biggest_city_size': 700000.0, 'num_cities': 3}
>>> c = Country.objects.filter(name='США').with_num_cities().with_biggest_city_size()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'QuerySet' object has no attribute 'with_num_cities'
>>>

>>> c = Country.objects.get(name='США').with_num_cities().with_biggest_city_size()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'Country' object has no attribute 'with_num_cities'
>>>
>>>
>>>
>>> c = City.objects.get(name='Москва').with_dencity()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'City' object has no attribute 'with_dencity'
>>>
>>>
>>> c = Country.objects.get(name='США').with_num_cities()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'Country' object has no attribute 'with_num_cities'
>>>