If you want to see what versions of Rails you have installed
gem list --local rails
If you want to install a version of Rails you don’t already have installed (assuming 2.2.3 is the version you don’t have installed)
gem install rails --version 2.2.3
If you want to start up your Rails app with a specific version (assumes you want to start up version 2.2.3)
rails _2.2.3_ --version
If you want to bind your Rails application to a specific Gem version of Rails
Edit your config/environment.rb file and modify the version of RAILS_GEM_VERSION to something else. For example you can set RAILS_GEM_VERSION = "2.2.3"
If you want to freeze / tie your Rails application to a specific version of Rails, do the following:
cd your_rails_app rake rails:freeze:gems
The above command copies the Rails files into your Rails’ application’s vendor/rails directory. When you start up Rails within your rails application directory, your application’s frozen version will be used instead of the your computer’s version.
To unfreeze your Rails version and go back to using your system-wide Gem version of Rails
cd your_rails_app rake rails:unfreeze
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