code: def _get_reference(document_path, reference_map):
      
      try:
          return reference_map[document_path]
      except KeyError:
          msg = _BAD_DOC_TEMPLATE.format(document_path)
          raise ValueError(msg)
docs: Get a document reference from a dictionary.
  
      This just wraps a simple dictionary look-up with a helpful error that is
      specific to :meth:`~.firestore.client.Client.get_all`, the
      **public** caller of this function.
  
      Args:
          document_path (str): A fully-qualified document path.
          reference_map (Dict[str, .DocumentReference]): A mapping (produced
              by :func:`_reference_info`) of fully-qualified document paths to
              document references.
  
      Returns:
          .DocumentReference: The matching reference.
  
      Raises:
          ValueError: If ``document_path`` has not been encountered.