If you're using bundler to manage your ruby gems, and capistrano to deploy your application, this handy deploy task can save you some time.

Add the following to deploy.rb

# ...lots of other code
namespace :bundle do

  desc "run bundle install and ensure all gem requirements are met"
  task :install do
    run "cd #{current_path} && bundle install  --without=test --no-update-sources"
  end

end
before "deploy:restart", "bundle:install"

After the code on the server has been updated, but before the application has been restarted, this task will run bundle install to ensure all gem requirements are met.

Note: "--without=test" and "--no-update-source" are optional. "--without=test" ignores any gems that are in the test group within your Gemfile and "--no-update-source" speeds things up a little by skipping the "Fetching source index for http://rubygems.org/..."