code: def _fallback(self, section_name, pref_name):
          
          message = (
              'Creating a fallback preference with ' +
              'section "{}" and name "{}".' +
              'This means you have preferences in your database that ' +
              'don\'t match any registered preference. ' +
              'If you want to delete these entries, please refer to the ' +
              'documentation: https://django-dynamic-preferences.readthedocs.io/en/latest/lifecycle.html')  # NOQA
          warnings.warn(message.format(section_name, pref_name))
  
          class Fallback(MissingPreference):
              section = Section(name=section_name) if section_name else None
              name = pref_name
              default = ''
              help_text = 'Obsolete: missing in registry'
  
          return Fallback()
docs: Create a fallback preference object,
          This is used when you have model instances that do not match
          any registered preferences, see #41