补全xml_id，如果没有自动新建，如果有就返回，

    def __ensure_xml_id(self, skip=False):
        """ Create missing external ids for records in ``self``, and return an
            iterator of pairs ``(record, xmlid)`` for the records in ``self``.

        :rtype: Iterable[Model, str | None]
        """

获取record的external_id

    def get_external_id(self):
        """Retrieve the External ID of any database record, if there
        is one. This method works as a possible implementation
        for a function field, to be able to add it to any
        model object easily, referencing it as ``Model.get_external_id``.

        When multiple External IDs exist for a record, only one
        of them is returned (randomly).