Django filter and annotate

Question

I have a table called User which is defined like this:

IAM_CHOICES = (
    ('HomeOwner', 'HomeOwner'),
    ('Architect', 'Architect'),
    ('Interior Designer', 'Interior Designer'),
)

class User(AbstractBaseUser, PermissionsMixin):
    first_name = models.CharField(_('First Name'), max_length=30, blank=True, null=True)
    middle_name = models.CharField(_('Middle Name'), max_length=30, blank=True, null=True)
    last_name = models.CharField(_('Last Name'), max_length=30, blank=True, null=True)
    about_yourself = models.TextField(null=True, blank=True)
    image = models.ImageField(upload_to=get_upload_path, null=True, blank=True)
    email = models.EmailField(_('Email Address'), unique=True,
        help_text=_('Please use your personal Email Address, not your Company or College'))
    address = models.TextField(null=True, blank=True)
    city = models.ForeignKey('common.City', null=True, blank=True)
    state = models.ForeignKey('common.State', null=True, blank=True)
    Iam = models.CharField(max_length=50, choices = IAM_CHOICES, null=True, blank=True)

and city table defined like this:

class City(models.Model):
    name = models.CharField(max_length=100)

How do i find city most users are from and are of certain Iam choice of ? i.e say for example find me the cities from where maximum architect (Iam choice type Architect) come from !

Answer 1

You can order the cities with maximum number of Architects like this -

City.objects.filter(user__Iam='Architect').annotate(num_user=Count('user')).orde‌​r_by('-num_user')

This first filters cities that have architects, then counts the architects and then order by them in descending order.