How to work with different versions of rails project at once?

With git is possible to jump back to early versions of a project.

Now, to work with these early versions, they depend often on old versions of libraries. Is it possible to circumvent problems like these:

/opt/local/lib/ruby/gems/1.8/gems/rails-2.2.2/lib/rails/gem_d开发者_如何学Pythonependency.rb:220:in
`specification': can't activate haml (= 2.1.0, runtime), 
  already activated haml-3.0.18 (Gem::Exception)

You can specify a version number with each of your gem requirements, either in your gemfile (Rails 3) or your environment files (Rails 2). As long as you don't uninstall any of the versions you're using, you should be able to switch back and forth as you please.