Python's Flask is a superb Sinatra clone, and Django's ActiveRecord equivalents are equally useful in Flask, especially for Mongo. Flask sits on top of Werkzeug

Python's Flask is a superb Sinatra clone, and Django's ActiveRecord equivalents are equally useful in Flask, especially for Mongo. Flask sits on top of Werkzeug, which is similar in power and use cases as Rack - middleware, fine-grain auth, etc.