Generate an application with a specific Rails version

Ok, so you are using rbenv to manage multiple ruby versions in your system. What if you want to create, let's say, both Rails 4.0 and Rails 3.2 applications using the same ruby version?

Instead of installing Rails with the command gem install ... we will use Bundler. How?

mkdir app
cd app
echo "source 'https://rubygems.org'" > Gemfile
echo "gem 'rails', '3.2.17'" >> Gemfile
bundle install

So now we have a minimal project with Rails 3.2.17 bundled. Now, we can use the rails command line tool with bundle exec to force the version:

bundle exec rails new . --force --skip-bundle
bundle update

Many thanks to Michael Trojanek (http://www.relativkreativ.at/articles/managing-multiple-rails-versions) who pointed me to the Right Way™.