code: def validate(self, value, model_instance):
          
          # raise validation error if the use of this field says it can't be blank but it is
          if not self.blank and value is self.Empty:
              raise ValidationError(self.error_messages['blank'])
          else:
              return super(OpaqueKeyField, self).validate(value, model_instance)
docs: Validate Empty values, otherwise defer to the parent