docs: def __get_value(self, field_name):
          """ Get request Json value by field name """
          value = request.values.get(field_name)
          if value is None:
              if self.json_form_data is None:
                  value = None
              elif field_name in self.json_form_data:
                  value = self.json_form_data[field_name]
          return value