ruby on rails - How to add a gem dependency and bundle install -

is possible add gem dependency rails 3 app programmatically through ruby code?

edit

i'm trying achieve when launch rake task or ruby script rails app becomes sass rails app.

i have bunch of file needs copied , after i'd add:

gem 'sass' 

to gemfile , run bundle install automatically.

today's railscasts features guard!

one of guard extensions guard-bundler has code can achieve need.

https://github.com/guard/guard-bundler/blob/master/lib/guard/bundler.rb

i thought there way perform similar tasks bundler class.