How to dual boot rails apps

When upgrading from one major rails release to the next it is very helpful to run your code base on both versions in parallel.

Currently rails 6 is just around the corner. Lets give it a try.

Installed gems and their versions are managed by bundler within the Gemfile.

To be able to install different sets of module versions we can use multiple Gemfile:

cp Gemfile Gemfile_rails6

This would duplicate everything. To avoid duplication we create an additional Gemfile.common to include gems that are common to both:

cp Gemfile Gemfile.common

Gemfile:

source 'https://rubygems.org'

# the gems that differ
gem 'rails', '~> 5.2'
gem 'ransack'

# common gems go there
eval_gemfile 'Gemfile.common'

Gemfile_rails6:

source 'https://rubygems.org'

# different gems required for rails6
gem 'rails', '6.0.0.rc2'
gem 'ransack', git: 'https://github.com/activerecord-hackery/ransack'

# common gems go there
eval_gemfile 'Gemfile.common'

All the rest goes to Gemfile.common.

Now you can switch between rails version using the BUNDLE_GEMFILE environment variable:

export BUNDLE_GEMFILE="Gemfile_rails6"
cp Gemfile.lock Gemfile_rails6.lock
bundle install

This will start with your existing gems versions and upgrade to rails 6.

You may skip the step of copying the lock file to start with the latest gems.

Docker environments

If you’re using a dockerized environment you can use it the following way.

Dockerfile:

ARG BUNDLE_GEMFILE
ENV BUNDLE_GEMFILE=${BUNDLE_GEMFILE:-Gemfile}
# COPY Gemfile*
# RUN bundle install

docker-compose.yml

version: '3'

services:
  web:
    build:
      args:
        - "BUNDLE_GEMFILE=${BUNDLE_GEMFILE}"
      context: .