Recently I wrote a Rails engine gem password_required. One of the first problems I ran into was not knowing what versions of Rails it would work with, so I kept the dependency rigid, “~> 4.1.6”. That of course alienated anyone working from the 4.0.x branch. I looked around at different packages to see how they handle this, and here is the solution I landed on.

First, ditch your Gemfile.lock and add it to your .gitignore, it will give you headaches when trying to bundle different versions of Rails for testing.

Next the following is found in my Gemfile..

rails_version = ENV['RAILS_VERSION'] || 'default'
rails = case rails_version
        when 'master'
          { github: 'rails/rails' }
        when 'default'
          '~> 4.1'
        else
          "~> #{rails_version}"
        end
gem 'rails', rails

And you will need to alter your specfile…

Gem::Specification.new do |s|
  s.add_dependency 'rails', '~> 4'
  # ...
end

This lets you do the following to test against rails 4.0 :

export RAILS_VERSION=4.0.0
bundle && rake
export RAILS_VERSION=default

The code could be better for this but you get the point. If you are using Travis to test this makes it pretty simple, here is my travis file env.

env:
  - "RAILS_VERSION=4.1.0"
  - "RAILS_VERSION=4.0.0"

Probably the biggest pain I had in this whole process was some of the errors I had to deal with in my spec/dummy rails testbed. Since I started with 4.1 there were boilerplate problems I ran into when trying to use 4.0. Here were the changes I had to make

# This doesn't work for rails 4.0 yo
# Rails.application.configure do
Dummy::Application.configure do

# This doesn't work for rails 4.0 yo
# Rails.application.configure do
Dummy::Application.configure do
  config.secret_key_base = 'xxxxx'

I tried briefly to go back as far as rails 3.2 but had a lot of issues so I stuck with 4.0 + for now. Hopefully this is helpful to anyone else who is looking to support multiple branches of rails in their gem.