ruby - How to switch to an older version of rails

Question

I'm trying to follow an older book written in 2011.

So, randomly picked 3.1.3 version since that's Nov 2011.

How do I switch to that version in my RVM?

Currently have Rails 4.0.0.beta1 version ...

Please don't suggest to put it in the Gemfile because there are a lot of other dependencies that will just be too troublesome to deal with, hence if I just switch, I'm assuming it means that other dependencies will be taken care of at the same time too, right?

UPDATE

**-s-MacBook-Pro:agile **$ rails -v
Rails 4.0.0.beta1
**-s-MacBook-Pro:agile **$ gem install rails -v 3.1.3
Successfully installed rails-3.1.3
1 gem installed
**-s-MacBook-Pro:agile **$ rvm gemset create rails 313
gemset created rails    => /Users/**/.rvm/gems/ruby-1.9.3-p392@rails
gemset created 313  => /Users/**/.rvm/gems/ruby-1.9.3-p392@313
**-s-MacBook-Pro:agile **$ ruby -v
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin10.8.0]
**-s-MacBook-Pro:agile **$ rvm 1.9.3p392-head@rails313
Unknown ruby interpreter version: '1.9.3p392'.
**-s-MacBook-Pro:agile **$ 

UPDATE AGAIN

**-s-MacBook-Pro:agile **$ rvm gemset use 313
Using ruby-1.9.3-p392 with gemset 313
**-s-MacBook-Pro:agile **$ rails -v
/Library/Ruby/Site/1.8/rubygems/dependency.rb:296:in `to_specs': Could not find 'rails'     (>= 0) among 2 total gem(s) (Gem::LoadError)
from /Library/Ruby/Site/1.8/rubygems/dependency.rb:307:in `to_spec'
from /Library/Ruby/Site/1.8/rubygems/core_ext/kernel_gem.rb:47:in `gem'
from /usr/bin/rails:18
**-s-MacBook-Pro:agile **$ 

Answer 1

First, uninstall the version of Rails you have:

gem uninstall rails

Next, install the version of Rails you want, like so:

gem install rails -v 3.1.12

There are a few ways to have both "installed" at the same time. As Joe Frambach suggested, you could install Rails 4 in a VM. You could also install RVM - the Ruby enVironment Manager - and use separate gemsets to keep the two versions of Rails apart. But if you are just learning you may not want to bother with this.

Edit: @Shadwell's answer got it right, although it could use some expansion, which I'll do here:

> rvm gemset create rails3
> rvm gemset use rails3
> gem install rails -v 3.1.12
> rails my_new_app