code: def validate(self, value, model_instance):
          
          if not self.multiple:
              return super(CountryField, self).validate(value, model_instance)
  
          if not self.editable:
              # Skip validation for non-editable fields.
              return
  
          if value:
              choices = [option_key for option_key, option_value in self.choices]
              for single_value in value:
                  if single_value not in choices:
                      raise exceptions.ValidationError(
                          self.error_messages["invalid_choice"],
                          code="invalid_choice",
                          params={"value": single_value},
                      )
  
          if not self.blank and value in self.empty_values:
              raise exceptions.ValidationError(self.error_messages["blank"], code="blank")
docs: Use custom validation for when using a multiple countries field.