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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _database_string(self):
 """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.) """

        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