How can I make cookies secure (https-only) by default in rails?

There’s no need to monkeypatch ActionController/ActionDispatch, and force_ssl has side effects (e.g. when behind an ELB).

The most straightforward way to achieve secure cookies is to modify config/initializers/session_store.rb:

MyApp::Application.config.session_store( 
  :cookie_store, 
  key: '_my_app_session',
  secure: Rails.env.production?
)