Rails, still?
"Sinatra is Ruby with Rails in mind. You will find yourself using more of Rails in your Sinatra application over time and ask yourself why you're not just using Rails."This is an important point. If you've ever developed a non-trivial application using Sinatra, Express or any other "micro" or "non-opinionated" framework, you'll have hit this wall quickly.To riff on Greenspun's Tenth Rule, "Any sufficiently complicated $MICRO-FRAMEWORK application contains an ad hoc informally-specified bug-ridden slow implementation of half of Ruby on Rails."