code: def _database_string(self):
          
          if self._database_string_internal is None:
              # NOTE: database_root_path() is a classmethod, so we don't use
              #       self._firestore_api (it isn't necessary).
              db_str = firestore_client.FirestoreClient.database_root_path(
                  self.project, self._database
              )
              self._database_string_internal = db_str
  
          return self._database_string_internal
docs: The database string corresponding to this client's project.
  
          This value is lazy-loaded and cached.
  
          Will be of the form
  
              ``projects/{project_id}/databases/{database_id}``
  
          but ``database_id == '(default)'`` for the time being.
  
          Returns:
              str: The fully-qualified database string for the current
              project. (The default database is also in this string.)