docs: def get_parents_graph(self, item_ids, language=None):
          """
          Get a subgraph of items reachable from the given set of items through
          the 'parent' relation.
  
          Args:
              item_ids (list): items which are taken as roots for the reachability
              language (str): if specified, filter out items which are not
                  available in the given language
  
          Returns:
              dict: item id -> list of items (parent items), root items are
              referenced by None key
          """
          def _parents(item_ids):
              if item_ids is None:
                  items = Item.objects.filter(active=True).prefetch_related('parents')
              else:
                  item_ids = [ii for iis in item_ids.values() for ii in iis]
                  items = Item.objects.filter(id__in=item_ids, active=True).prefetch_related('parents')
              return {item.id: sorted([_item.id for _item in item.parents.all()]) for item in items}
          return self._reachable_graph(item_ids, _parents, language=language)
  
          if item_ids is None:
              return self._reachable_graph(None, _parents, language=language)
          else:
              graph = self.get_parents_graph(None, language)
              return self._subset_graph(graph, item_ids)