code: def url_id(cls):
          
          if cls.__uuid_primary_key__:
              def url_id_func(self):
                  """The URL id, UUID primary key rendered as a hex string"""
                  return self.id.hex
  
              def url_id_is(cls):
                  return SqlHexUuidComparator(cls.id)
  
              url_id_func.__name__ = 'url_id'
              url_id_property = hybrid_property(url_id_func)
              url_id_property = url_id_property.comparator(url_id_is)
              return url_id_property
          else:
              def url_id_func(self):
                  """The URL id, integer primary key rendered as a string"""
                  return six.text_type(self.id)
  
              def url_id_expression(cls):
                  """The URL id, integer primary key"""
                  return cls.id
  
              url_id_func.__name__ = 'url_id'
              url_id_property = hybrid_property(url_id_func)
              url_id_property = url_id_property.expression(url_id_expression)
              return url_id_property
docs: The URL id