class ZipCode(models.Model):
    zip = models.CharField(max_length=20)
    cities = City.objects.filter(zip=self).distinct()

class City(models.Model):
    name = models.CharField(max_length=50)
    slug = models.CharField(max_length=50)
    state = models.ForeignKey(State)
    zip = models.ManyToManyField(ZipCode)

NameError: name 'City' is not defined

cities = City.objects.filter(zip=self).distinct()

cities = zip.city_set.all()

cities = City.objects.filter(zip=self).distinct()

@property
def cities(self):
  return City.objects.filter(zip=self).distinct()

a_zip_code.city_set.all()

zip = models.ManyToManyField(ZipCode, related_name='cities')
...
a_zip_code.cities.all()

attribute = models.ForeignKey('ClassDefinedAfterThis')

models.ForeignKey('appName.modelName')

cities = models.ForeignKey(City)

cities = models.ManyToManyField(City)