Upgrade Rails from 4.2 to 5.0

This article is part of our Upgrade Rails series. To see more of them, click here opens a new window .

This article will cover the most important aspects that you need to know to get your Ruby on Rails opens a new window application from version 4.2 opens a new window to 5.0 opens a new window .

1. Preparations
2. Ruby version
3. Gems
4. Config files (config/)
5. Application code
6. ActiveRecord
7. Controllers
8. Testing
9. Next steps

1. Preparations

Before beginning with the upgrade process, we have some recommended preparations:

• Your Rails app should have the latest patch version opens a new window before you move to the next major/minor version.
• You should have at least 80% test coverage unless you have a dedicated QA team.
• Follow a Git flow workflow to actively manage at least two environments: staging and production.
• Check your Gemfile.lock for incompatibilities by using RailsBump opens a new window .
• Create a dual boot mechanism, the fastest way to do this is installing the handy gem next_rails opens a new window .

For full details check out our article on How to Prepare Your App for a Rails Upgrade opens a new window .

2. Ruby version

Rails 5.0 opens a new window requires Ruby 2.2.2 opens a new window or later.

This Ruby upgrade shouldn’t generate any problems. However, if you run into this exception in your test suite:

cannot load such file -- test/unit/assertions (Load Error)

then you’ll need to add:

gem 'test-unit'

to your Gemfile.

3. Gems

• It’s recommended that you check your Gemfile against RailsBump opens a new window to ensure all your gems are compatible with Rails 5. As of the release of this blog post, there are only a few gems which don’t support Rails 5 yet. This is more of a problem when you’re upgrading early on.

If any of the gems are missing on RailsBump, you’ll need to manually check the Github page for the project to find out its status. In case you own the gem, you’ll need to make sure it works on Rails 5 or update it.

4. Config files

Rails includes the rails app:update task opens a new window . You can use this task as a guideline as explained thoroughly in this post opens a new window .

As an alternative, check out RailsDiff opens a new window , which provides an overview of the changes in a basic Rails app between 4.2.x and 5.0.x (or any other source/target versions).

5. Application code

a. ActiveRecord

• ActiveRecord models will now inherit from ApplicationRecord by default instead of ActiveRecord::Base. You should create an application_record.rb file under app/models with:

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

And then update all of your models to inherit from ApplicationRecord instead of ActiveRecord::Base. The only class that inherits from ActiveRecord::Base will be ApplicationRecord.

• belongs_to associations are now required by default opens a new window . This means that if the association doesn’t exist for a model when saving it, an error will be triggered. You can turn this feature off for an association by using optional: true:

belongs_to :user, optional: true

• ActiveRecord migrations now need to be tagged with the Rails version they are created under. One of the reasons for this is that strings in Rails 4.2 had a default size of 4 bytes. In Rails 5.0, their default size is now of 8 bytes. See this comment opens a new window from Rails core team member Rafael França regarding this change.

To resolve this, you’ll need to update your current migrations in your db/migrate directory:

class CreatePosts < ActiveRecord::Migration

class CreatePosts < ActiveRecord::Migration[4.2]

If you add a new migration after updating to Rails 5.0, you’ll also need to tag them appropriately:

class CreateProducts < ActiveRecord::Migration[5.0]

This will help with potential compatibility issues, so your database can be correctly reconstructed from the migrations. Strings in your 4.2 migrations will use the previous default size of 4 bytes instead of the new default.

Note you shouldn’t add the patch version to the tag, just the major and minor version numbers (4.2, 5.0, 5.1, 5.2).

(Thanks to Cory McDonald in the comments section for reminding us about this important change!)

b. Controllers

• If you’re not already using strong parameters, and still rely on protected_attributes, you should migrate your application to strong parameters before attempting to upgrade to Rails 5.0. protected_attributes opens a new window is no longer supported on Rails 5.0.

A few months ago we worked on a project to attempt to automate the strong parameters migration process, and this resulted in the gem RailsUpgrader opens a new window . It’s in a beta state, but you can try using it if you have too many models, or at least as a guide for a WIP branch.

There are still efforts being made to keep protected_attributes alive though, like the protected_attributes_continued opens a new window gem. I would strongly recommend against using it since its support is limited, and it won’t work in future Rails versions.

• Parameters now behave differently, they no longer inherit from HashWithIndifferentAccess. Some methods (e.g.: fetch, slice, except) you may be now calling on params will no longer work. You will need to permit the parameters, call to_h, and only then you’ll be able to run the Hash methods you need on them.

For more information: https://github.com/rails/rails/pull/20868 opens a new window

6. Testing

• One of the most common methods in controller tests, assigns, has been extracted from Rails and moved to the rails-controller-testing opens a new window gem. If you wish to continue using it (and assert_template), you’ll need to add it to your Gemfile:

gem 'rails-controller-testing'

• Instead of using ActionDispatch::Http::UploadedFile to test uploads, you’ll need to update those tests to use Rack::Test::UploadedFile opens a new window .

7. Next steps

If you successfully followed all of these steps, you should now be running Rails 5.0! Do you have any other useful tips or recommendations? Share them with us in the comments section.

If you’re not on Rails 5.0 yet, we can help! Download our free eBook: The Complete Guide to Upgrade Rails opens a new window .