Specifying rails version to use when creating a new application

Question

I found here an undocumented option to create a new application using an older version of Rails.

rails _2.1.0_ new myapp 

Here is the command which I use normally:

rails _version_ new application_name

for example rails _2.1.0_ new my_app

Here is the list of all available rails versions so far:

http://rubygems.org/gems/rails/versions

I was having some trouble using rails _version_ new application_name (the resulting project was still generated for the newest version of Rails installed.)

After a bit of digging I found an article by Michael Trojanek with an alternative approach. This works by creating a folder with a Gemfile specifying the desired version of Rails and then using bundle exec rails... so that Bundler takes care of running the appropriate version of rails. e.g. to make a new Rails 4.2.9 projects the steps are:

mkdir myapp
cd myapp
echo "source 'https://rubygems.org'" > Gemfile
echo "gem 'rails', '4.2.9'" >> Gemfile
bundle install

bundle exec rails new . --force --skip-bundle
bundle update

As rightly pointed out by @mikej for Rails 5.0.0 or above, you should be following these steps:

Create a directory for your application along with a Gemfile to specify your desired Rails version and let bundler install the dependent gems:

$ mkdir myapp
$ cd myapp
$ echo "source 'https://rubygems.org'" > Gemfile
$ echo "gem 'rails', '5.0.0.1'" >> Gemfile
$ bundle install

Check that the correct version of rails has been installed: $ bundle exec rails -v

Now create your application, let Rails create a new Gemfile (or rather overwrite the existing one by using the --force flag) and instead of installing the bundle (--skip-bundle) update it manually:

$ bundle exec rails new . --force --skip-bundle

If you check the entry for rails in Gemfile, it should be like this:

gem 'rails', '~> 5.0.0', '>= 5.0.0.1'

You should update it to the exact version needed for the application:

gem 'rails', '5.0.0.1'

Now, the final step:

$ bundle update