old_contents: # coding=utf-8
  
  from django.db import models
  from django.utils.translation import ugettext as _
  
  
  class VKPlace(models.Model):
  
      """Abstract model for VK.com places."""
  
      vk_id = models.PositiveIntegerField(editable=False, unique=True)
      name = models.CharField(_('Name'), max_length=255)
  
      class Meta:
          abstract = True
  
      def __str__(self):
          return self.name
  
  
  class Country(VKPlace):
  
      """VK.com country."""
  
      class Meta:
          verbose_name = _('Country')
          verbose_name_plural = _('Countries')
  
  
  class Region(VKPlace):
  
      """VK.com country region."""
  
      country = models.ForeignKey(
          Country, related_name='regions', verbose_name=_('Country')
      )
  
      class Meta:
          verbose_name = _('Region')
          verbose_name_plural = _('Regions')
  
  
  class City(VKPlace):
  
      """VK.com city."""
  
      region = models.ForeignKey(
          Region, related_name='cities', verbose_name=_('Region')
      )
      area = models.CharField(_('Area'), max_length=255, null=True, blank=True)
  
      class Meta:
          verbose_name = _('City')
          verbose_name_plural = _('Cities')
  
new_contents: # coding=utf-8
  
  from django.db import models
  from django.utils.translation import ugettext as _
  
  
  class VKPlace(models.Model):
  
      """Abstract model for VK.com places."""
  
      vk_id = models.PositiveIntegerField(editable=False, unique=True)
      name = models.CharField(_('Name'), max_length=255)
  
      class Meta:
          abstract = True
  
      def __str__(self):
          return self.name
  
  
  class Country(VKPlace):
  
      """VK.com country."""
  
      class Meta:
          verbose_name = _('Country')
          verbose_name_plural = _('Countries')
  
  
  class Region(VKPlace):
  
      """VK.com country region."""
  
      country = models.ForeignKey(
          Country, related_name='regions', on_delete=models.CASCADE, verbose_name=_('Country')
      )
  
      class Meta:
          verbose_name = _('Region')
          verbose_name_plural = _('Regions')
  
  
  class City(VKPlace):
  
      """VK.com city."""
  
      region = models.ForeignKey(
          Region, related_name='cities', on_delete=models.CASCADE, verbose_name=_('Region')
      )
      area = models.CharField(_('Area'), max_length=255, null=True, blank=True)
  
      class Meta:
          verbose_name = _('City')
          verbose_name_plural = _('Cities')
  
subject: Update models.py