def update(self, document_id, update_spec, namespace, timestamp):
        """Apply updates given in update_spec to the document whose id
        matches that of doc.

        """
        document = self.doc_dict[document_id].doc
        updated = self.apply_update(document, update_spec)
        if "_id" in updated:
            updated.pop("_id")
        updated[self.unique_key] = document_id
        self.upsert(updated, namespace, timestamp)
        return updated
Apply updates given in update_spec to the document whose id
        matches that of doc.