Minor factorization.

        Updates are coalesced so that we only run one update query per ancestor.

        """

        try:

            self.related_updates[model].append((field, None, value))

        except KeyError:

            self.related_updates[model] = [(field, None, value)]

        self.related_updates.setdefault(model, []).append((field, None, value))

    def get_related_updates(self):

        """