code: def _document_path(self):
          
          if self._document_path_internal is None:
              if self._client is None:
                  raise ValueError("A document reference requires a `client`.")
              self._document_path_internal = _get_document_path(self._client, self._path)
  
          return self._document_path_internal
docs: Create and cache the full path for this document.
  
          Of the form:
  
              ``projects/{project_id}/databases/{database_id}/...
                    documents/{document_path}``
  
          Returns:
              str: The full document path.
  
          Raises:
              ValueError: If the current document reference has no ``client``.