code: def _get_modcommit(self, key):
          
          try:
              return ModificationCommit.objects.filter(
                  object_uid = self.uid,
                  key = key,
                  action__id__lte = self.step
              ).order_by("-action__id")[0]
          except IndexError:
              return None
docs: Return the last modcommit of the given field. If no
          modcommit exists (for example after a migration that created
          new fields) returns None.