Write back documents by doc_id

        :param documents: a list of document to write back
        :param doc_ids: a list of document IDs which need to be written back
        :returns: a list of document IDs that have been written

def write_back(self, documents, doc_ids=None, eids=None):
        """
        Write back documents by doc_id

        :param documents: a list of document to write back
        :param doc_ids: a list of document IDs which need to be written back
        :returns: a list of document IDs that have been written
        """
        doc_ids = _get_doc_ids(doc_ids, eids)

        if doc_ids is not None and not len(documents) == len(doc_ids):
            raise ValueError(
                'The length of documents and doc_ids is not match.')

        if doc_ids is None:
            doc_ids = [doc.doc_id for doc in documents]

        # Since this function will write docs back like inserting, to ensure
        # here only process existing or removed instead of inserting new,
        # raise error if doc_id exceeded the last.
        if len(doc_ids) > 0 and max(doc_ids) > self._last_id:
            raise IndexError(
                'ID exceeds table length, use existing or removed doc_id.')

        data = self._read()

        # Document specified by ID
        documents.reverse()
        for doc_id in doc_ids:
            data[doc_id] = dict(documents.pop())

        self._write(data)

        return doc_ids