code: def _get_ancestors_path(self, model, levels=None):
          
          if not issubclass(model, self.model):
              raise ValueError(
                  "%r is not a subclass of %r" % (model, self.model))
  
          ancestry = []
          # should be a OneToOneField or None
          parent_link = model._meta.get_ancestor_link(self.model)
          if levels:
              levels -= 1
          while parent_link is not None:
              related = parent_link.remote_field
              ancestry.insert(0, related.get_accessor_name())
              if levels or levels is None:
                  parent_model = related.model
                  parent_link = parent_model._meta.get_ancestor_link(
                      self.model)
              else:
                  parent_link = None
          return LOOKUP_SEP.join(ancestry)
docs: Serves as an opposite to _get_subclasses_recurse, instead walking from
          the Model class up the Model's ancestry and constructing the desired
          select_related string backwards.