Confusion on how the bundler's group works - ruby

Question

I'm using ruby 1.9.3p194 and bundler 1.1.4
In my Gemfile I have this:
group :production do
gem 'thin', '1.4.1'
end
When I run $ rails s, bundler keeps complains:
Could not find gem 'thin (>= 1.4.1) ruby' in the gems available on this machine.
Run `bundle install` to install missing gems.
I'm running under development environment, so shouldn't it NOT care if it is installed or not? Dose bundler force you to install ALL the gems when running $ rails s?
I also tried the group:test, same thing happens. That doesn't really make sense to me, can anyone help?
my gemfile:
source 'https://rubygems.org'
gem 'rails', '3.2.6'
gem 'mysql2'
# Gems used only for assets and not required
# in production environments by default.
group :assets do
gem 'sass-rails', '~> 3.2.3'
gem 'coffee-rails', '~> 3.2.1'
# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer', :platforms => :ruby
gem 'uglifier', '>= 1.0.3'
end
gem 'jquery-rails'
group :production do
gem 'thin', '1.4.1'
end
gem 'devise', '2.1.2'
gem 'cancan', '1.6.8'
gem 'will_paginate', '3.0.3'

Answer 1

Possibly the problem is that you are running rails s, while you need to run bundle exec rails s. If you precede a command with bundle exec, it is executed within the context of the bundle. All the gems available to bundler are then available to the command.
By default, gems from bundles are installed in the global gem directory. This can produce confusing results when you don't prepend bundle exec to a command: imagine you have both rails 3.1 and rails 3.2 in your global gems and your Gemfile mentions 3.1. Then you will still call the executable from rails 3.2 when you simply say rails on the command line.
Now of course thin could only be missing if it wasn't installed with the global gems. Assuming you've used the --path option to bundler at least once, your gems have been installed to the path specified there and not to the global repository. After using --path, you must specify bundle exec, otherwise gems simply won't be found at all.