The new Ruby 1.9.3 breaks the old Ruby Debugger. Following steps are needed to get it work again. Furthermore, Mongoid 3.0.x requires Ruby 1.9.3. The old Ruby 1.9.3 does not work. Below is a working shell script

echo "Installing ruby-debug with ruby-1.9.3-p0 ..."

curl -OL http://rubyforge.org/frs/download.php/75414/linecache19-0.5.13.gem
curl -OL http://rubyforge.org/frs/download.php/75415/ruby-debug-base19-0.11.26.gem
sudo gem install linecache19-0.5.13.gem ruby-debug-base19-0.11.26.gem -- --with-ruby-include=$rvm_path/src/ruby-1.9.3-p194/
sudo gem install --pre ruby-debug-base19x

echo "Done."