docs: def _view_filter(self):
          """
          Returns the mongodb catalog filter for isolated or federated views.
  
          This also searches across all underlying catalogs in federated
          catalog views. Real authz for controlling access to underlying
          catalogs will need to be managed in an adapter above the
          pay grade of this implementation.
  
          """
          if self._is_phantom_root_federated():
              return {}
          idstr_list = self._get_catalog_idstrs()
          return {'assigned' + self._catalog_name + 'Ids': {'$in': idstr_list}}