Pry is the way to go for debugging in Ruby. Drop into an interactive REPL with local variable context in situ with binding.pry or binding.pry_remote if in a forked server environment.

For subtle edits of a big chunk of code, my preferred approach is to set everything up in a test case, then pop into the implementation that needs to be changed where all the state is available with a binding.pry, and iterate expressions in the REPL until it's good, then copy my history out into an editor.

In Emacs it's nice and easy with rspec-mode and inf-ruby - run the test from within Emacs and get in-editor REPL once you hit the binding.pry.