docs: def down(self, migration_id):
          """Rollback to migration."""
          if not self.check_directory():
              return
  
          for migration in self.get_migrations_to_down(migration_id):
              logger.info('Rollback migration %s' % migration.filename)
  
              migration_module = self.load_migration_file(migration.filename)
              if hasattr(migration_module, 'down'):
                  migration_module.down(self.db)
              else:
                  logger.info('No down method on %s' % migration.filename)
  
              self.collection.remove({'filename': migration.filename})