Return the last modcommit of the given field. If no
        modcommit exists (for example after a migration that created
        new fields) returns None.

def _get_modcommit(self, key):
        """Return the last modcommit of the given field. If no
        modcommit exists (for example after a migration that created
        new fields) returns None.
        """
        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