How to remove gem from Ruby on Rails application?

I have installed a gem on my Rails application (devise). After I installed the gem, I realized that I don't need it.

I want to remove the gem, its dependencies and the files it created on my application. In other words, I want to restore the system to what it used to be before the gem. How can I do this? (I'm using Ruby on Rails 3.)

You can use

gem uninstall <gem-name>

If you're using Rails 3+, remove the gem from the Gemfile and run bundle install.

If you're using Rails 2, hopefully you've put the declaration in config/environment.rb. If so, removing it from there and running rake gems:install should do the trick.

Devise uses some generators to generate views and stuff it needs into your application. If you have run this generator, you can easily undo it with

rails destroy <name_of_generator>

The uninstallation of the gem works as described in the other posts.

For Rails 4 - remove the gem name from Gemfile and then run bundle install in your terminal. Also restart the server afterwards.

How about something like:

gem dependency devise --pipe | cut -d \  -f 1 | xargs gem uninstall -a

(this assumes that you're not using bundler - but I guess you're not since removing from your bundle gemspec would solve the problem)