code: def get_all(self, references, field_paths=None, transaction=None):
          
          document_paths, reference_map = _reference_info(references)
          mask = _get_doc_mask(field_paths)
          response_iterator = self._firestore_api.batch_get_documents(
              self._database_string,
              document_paths,
              mask,
              transaction=_helpers.get_transaction_id(transaction),
              metadata=self._rpc_metadata,
          )
  
          for get_doc_response in response_iterator:
              yield _parse_batch_get(get_doc_response, reference_map, self)
docs: Retrieve a batch of documents.
  
          .. note::
  
             Documents returned by this method are not guaranteed to be
             returned in the same order that they are given in ``references``.
  
          .. note::
  
             If multiple ``references`` refer to the same document, the server
             will only return one result.
  
          See :meth:`~.firestore_v1beta1.client.Client.field_path` for
          more information on **field paths**.
  
          If a ``transaction`` is used and it already has write operations
          added, this method cannot be used (i.e. read-after-write is not
          allowed).
  
          Args:
              references (List[.DocumentReference, ...]): Iterable of document
                  references to be retrieved.
              field_paths (Optional[Iterable[str, ...]]): An iterable of field
                  paths (``.``-delimited list of field names) to use as a
                  projection of document fields in the returned results. If
                  no value is provided, all fields will be returned.
              transaction (Optional[~.firestore_v1beta1.transaction.\
                  Transaction]): An existing transaction that these
                  ``references`` will be retrieved in.
  
          Yields:
              .DocumentSnapshot: The next document snapshot that fulfills the
              query, or :data:`None` if the document does not exist.