Upgrading projects to rails 6

Update ruby to 2.6.6 and install bundle.

rvm install 2.6.6
gem install bundler

Update Gemfile

# Gemfile

gem 'rails', '~> 6.0.2.2'
gem 'pg', '~> 1.2.3'

Update test environment

# config/environments/test.rb

config.cache_classes = false

Remove all references to autoloaded paths from initializers! Do not i.e. set values based on Folio::Site.instance, as it breaks zeitwerk autoloading.

Rename decorators to overrides and add setup to application.rb to work with zeitwerk.

cd root_of_the_project || exit 1
mv app/decorators app/overrides
for f in app/overrides/**/*_decorator.rb; do newname="${f/_decorator.rb/_override.rb}"; mv $f $newname; done

# config/application.rb

#...
module Dummy
  class Application < Rails::Application
    #..
    config.load_defaults 6.0
    #...
    Rails.autoloaders.main.ignore("#{::Folio::Engine.root}/app/lib/folio/console/simple_form_components")
    Rails.autoloaders.main.ignore("#{::Folio::Engine.root}/app/lib/folio/console/simple_form_inputs")

    overrides = [
      Folio::Engine.root.join('app/overrides').to_s,
      Rails.root.join('app/overrides').to_s,
    ]

    overrides.each { |override| Rails.autoloaders.main.ignore(override) }

    config.to_prepare do
      overrides.each do |override|
        Dir.glob("#{override}/**/*_override.rb").each do |file|
          load file
        end
      end

      Dir.glob("#{::Folio::Engine.root}/app/lib/folio/console/simple_form*/**.rb").each do |sf|
        load sf
      end
    end
  end
end

Change rvm_ruby_version in deploy.rb

# config/deploy.rb

set :rvm_ruby_version, '2.6.6'

Change TargetRubyVersion in rubocop config

# .rubocop.yml

AllCops:
  TargetRubyVersion: 2.6.6