What I want to install

ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin12.2.0]
Rails 3.2.11

1. Install rbenv and ruby-build

$ brew install rbenv
$ brew install ruby-build

Here, to load rbenv when bash starts,

eval "$(rbenv init -)"

add this to ~/.bash_profile.

2. Install Ruby

$ rbenv install -l    # List of installable Ruby versions
Available versions:
 1.8.6-p383
 1.8.6-p420
 1.8.7-p249
 1.8.7-p302
 1.8.7-p334
 1.8.7-p352
 1.8.7-p357
 1.8.7-p358
 1.8.7-p370
 1.8.7-p371
 1.9.1-p378
 1.9.2-p180
 1.9.2-p290
 1.9.2-p318
 1.9.2-p320
 1.9.3-dev
 1.9.3-p0
$ rbenv install 1.9.3-p194 # Install Ruby
$ rbenv global 1.9.3-p194  # Set default Ruby to 1.9.3

3. Install Rails

$ rbenv exec gem install rails --version="~> 3.2.11"
$ rbenv rehash

4. Verify

$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin12.3.0]
$ rails -v
Rails 3.2.12