code: def _upgrade_db(self):
          
          migration_path = "_data/migrations"
          sqlite3.connect(self._db_path).close()    # ensure that it exists
          db_url = "sqlite:///" + self._db_path
          backend = yoyo.get_backend(db_url)
          migration_dir = pkg_resources.resource_filename(__package__, migration_path)
          migrations = yoyo.read_migrations(migration_dir)
          assert len(migrations) > 0, "no migration scripts found -- wrong migraion path for " + __package__
          migrations_to_apply = backend.to_apply(migrations)
          backend.apply_migrations(migrations_to_apply)
docs: upgrade db using scripts for specified (current) schema version