code: def validate_unique(self, *args, **kwargs):
          
  
          super(BaseVersionedModel, self).validate_unique(*args, **kwargs)
          if hasattr(self, 'versioned_unique'):
              errors = {}
  
              for field in self.versioned_unique:
                  lookup_kwargs = {field: getattr(self, field)}
                  qs = self.__class__._default_manager.filter(**lookup_kwargs)
  
                  # Exclude the current object from the query if we are editing
                  # an instance (as opposed to creating a new one)
                  if self.object_id:
                      qs = qs.exclude(object_id=self.object_id)
  
                  if qs.exists():
                      msg = self.unique_error_message(self.__class__, (field,))
                      errors[field] = msg
  
              if errors:
                  raise ValidationError(errors)
docs: Calls super validate_unique and, after that is done, runs
          through any field_names listed in `self.versioned_unique`
          and checks that this is the only item with this name.
  
          These checks contain race conditions since it all happens
          before saves and so should not be relied upon for uniqueness
          but help with form validation.