Gemfile

source "http://rubygems.org"
gemspec

gem 'rake'

rails = ENV['RAILS'] || '4-2-stable'
arel = ENV['AREL'] || '6-0-stable'

arel_opts = case arel
when /\// # A path
  {:path => arel}
when /^v/ # A tagged version
  {:git => 'git://github.com/rails/arel.git', :tag => arel}
else
  {:git => 'git://github.com/rails/arel.git', :branch => arel}
end

gem 'arel', arel_opts

case rails
when /\// # A path
  gem 'activesupport', :path => "#{rails}/activesupport"
  gem 'activemodel', :path => "#{rails}/activemodel"
  gem 'activerecord', :path => "#{rails}/activerecord"
when /^v/ # A tagged version
  git 'git://github.com/rails/rails.git', :tag => rails do
    gem 'activesupport'
    gem 'activemodel'
    gem 'activerecord'
  end
else
  git 'git://github.com/rails/rails.git', :branch => rails do
    gem 'activesupport'
    gem 'activemodel'
    gem 'activerecord'
  end
end