text: class ModelFactory(object):
      model_name = None
  
      def __call__(self, manager):
          """
          Create model class but only if it doesn't already exist
          in declarative model registry.
          """
          Base = manager.declarative_base
          try:
              registry = Base.registry._class_registry
          except AttributeError:  # SQLAlchemy < 1.4
              registry = Base._decl_class_registry
          if self.model_name not in registry:
              return self.create_class(manager)
          return registry[self.model_name]
  