docs: def get_model_from_settings(settings_key):
      """Return the django model from a settings key.
  
      This is the same pattern user for django's "get_user_model()" method. To
      allow you to set the model instance to a different model subclass.
  
      :param settings_key: the key defined in settings to the value for
  
      """
      cls_path = getattr(settings, settings_key, None)
      if not cls_path:
          raise NotImplementedError()
  
      try:
          app_label, model_name = cls_path.split('.')
      except ValueError:
          raise ImproperlyConfigured("{0} must be of the form "
                                     "'app_label.model_name'".format(settings_key))
  
      model = apps.get_model(app_label, model_name)
  
      if model is None:
          raise ImproperlyConfigured("{0} refers to model '%s' that has not "
                                     "been installed".format(settings_key))
  
      return model