code: def filterAcceptsRow(self, row, parentindex):
          if not super(ReftrackSortFilterModel, self).filterAcceptsRow(row, parentindex):
              return False
          if parentindex.isValid():
              m = parentindex.model()
          else:
              m = self.sourceModel()
          i = m.index(row, 18, parentindex)
          reftrack = i.data(REFTRACK_OBJECT_ROLE)
          if not reftrack:
              return True
          else:
              return self.filter_accept_reftrack(reftrack)