code: def _update_cache(self):
          
          fields = connections[self.connection_alias].get_unified_index().all_searchfields()
          if self._fields != fields:
              self._fields = fields
              self._content_field_name, self._schema = self.build_schema(self._fields)
docs: To avoid build_schema every time, we cache
          some values: they only change when a SearchIndex
          changes, which typically restarts the Python.