Getting Ready for Rails 6.0: How to Dual Boot

Getting Ready for Rails 6.0: How to Dual Boot

In this article I will explain how you can dual boot your application in your local environment and your continuous integration (CI) service. I hope that this will help you get ready for the next stable release of Rails.

Even though my example assumes you are running Rails 5.2 opens a new window and want to migrate to Rails 6.0 opens a new window , these tips work for any two versions of Rails.

Create a File

At RailsConf 2018, Jordan Raine opens a new window talked about Clio’s process to upgrade Rails over the years. If you missed his talk, you can watch it over here: Ten Years of Rails Upgrades opens a new window

In his talk he mentioned quite a handy companion gem: ten_years_rails opens a new window . At opens a new window we decided to fork it and call it next_rails opens a new window . I’m going to use that gem to get my project ready for dual booting. First, I need to install it in my local environment.

$ gem install next_rails
Successfully installed next_rails-1.0.2
Parsing documentation for next_rails-1.0.2
Installing ri documentation for next_rails-1.0.2
Done installing documentation for next_rails after 0 seconds
1 gem installed

Warning: next_rails requires Ruby 2.3 or higher. You can find a manual workaround below.

Assuming I can use that gem, I will initialize my file like this:

$ next --init
Created (a symlink to your Gemfile). Your Gemfile has been modified
to support dual-booting!

There's just one more step: modify your Gemfile to use a newer version of Rails
using the `next?` helper method.

For example, here's how to go from 5.2.3 to 6.0:

if next?
  gem "rails", "6.0.0"
  gem "rails", "5.2.3"

That command creates a symlink opens a new window to my Gemfile and adds a handy method called next? to my Gemfile:

Why to use a symlink

I see three main benefits to using a symlink:

  1. The way Bundler works it will generate one .lock file per Gemfile. If you manage all your dependencies logic in your Gemfile (without and your Gemfile.lock is checked in to your Git repository, then you will have to constantly resolve conflicts between your long running upgrade branch and master. This will become tedious if you have a really active master branch and your upgrade project lasts months (not weeks)

  2. By making it a symlink to Gemfile, you can keep all your logic inside one file. That means that you can quickly see what are the main difference between your current version of Rails and the next version.

  3. You can use Bundler’s BUNDLE_GEMFILE environment variable. Because a symlink is transparent to Bundler, it assumes that you have two physical files. You can later switch between one version of Rails or the other by just adding one environment variable to your command line.

# Gemfile
def next?
  File.basename(__FILE__) == ""

source ''
# ...

If you have any problems installing next_rails, you can manually add the next? method to your Gemfile and create a symlink like this:

$ cd path/to/project
$ ln -s Gemfile

Bump Rails (

In this simple example, I only need to upgrade rails (from Rails 5.2 to Rails 6.0). It’s very likely that you will have to upgrade more dependencies. The first step is to get my Gemfile to look like this:

def next?
  File.basename(__FILE__) == ""

source ''
if next?
  gem 'rails', '~> 6.0.0'
  gem 'rails', '~> 5.2.3'

# ...

Now I can install my current dependencies with bundle install and my future dependencies with next bundle update. If next bundle update doesn’t work for you, you can just run bundle install. As a general rule, if next <command> doesn’t work in your environment you can replace it with <command>.

next bundle <command> might not work because you are using an old version of Bundler opens a new window . So, you should try using Bundler 2.0 or higher.

Run Tests

After running next bundle update, I have a brand new file. That means that my dependencies are ready to run my test suite. So I can run them like this:

$ next bundle exec rake

There are many advantages to using dual booting in your Rails application. In no particular order:

  • You can run your test suite with two different versions of Rails. Running bundle exec rake still works thanks to the conditionals in your Gemfile.
  • You can run your application in development with two different versions of Rails. Simply prepend next to bundle exec rails server.
  • You can even run your application in staging using the next version of Rails. Simply make sure that you set this environment variable: BUNDLE_GEMFILE
  • You can quickly debug issues between your current version of Rails and the next one. Dual booting plus debugger is a powerful combo for finding bugs between versions.

Setup Continuous Integration

Depending on the type of project, we like to use Travis CI opens a new window for open source projects and Circle CI opens a new window for client projects. Below you will find a couple of sample configuration files that you could use.

Both samples will require you to commit and push to your repository; will run a build matrix; and might need some tweaking.

Circle CI

For all of our client projects we prefer this continuous integration service. Here is the configuration that you could use for dual booting in Circle CI:

A few notes about this configuration:

  version: 2
      - "build-rails-5-2"
      - "build-rails-6-0"

Travis CI

For all of our open source projects we prefer this continuous integration service. Here is the configuration that you could use for dual booting in Travis CI:

A few notes about this configuration:

  • It’s for an open source application that is using Postgres
  • You might not need some of the things in this sample
  • It’s certainly simpler than the Circle CI configuration. I love how simple it is to configure a build matrix in Travis CI:
- 2.2.6
- Gemfile

Start Fixing The Rails 6.0 Test Suite

Now that you have your test suite running in both Rails 5.2 and Rails 6.0, you can start tweaking your code and dependencies to work with both gemfiles. There will be two big hurdles:

  1. Getting Bundler to bundle your dependencies
  2. Getting your test suite to pass


This has been quite a useful technique for us at opens a new window . We have used it with client projects, internal projects, and open source applications.

I hope that you will find it useful in getting ready for the next version of Rails!

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