A map of operation index to the _id of the upserted document.

def upserted_ids(self):
        """A map of operation index to the _id of the upserted document."""
        self._raise_if_unacknowledged("upserted_ids")
        if self.__bulk_api_result:
            return dict((upsert["index"], upsert["_id"])
                        for upsert in self.bulk_api_result["upserted"])