rails-upgrade-3-to-4

Rails 3 → 4 Upgrade

Legacy upgrade. Rails 3.2 is past EOL (Rails 3 LTS exists but is paid). The 3.2 → 4.0 hop is API-breaking; budget for a real project, not a weekend.

The opinion

Dual-boot with next_rails. Hop 3.2 → 4.0 → 4.1 → 4.2. Ruby 2.0+ required for 4.0; Ruby 2.2.2+ for 4.2. Use the protected_attributes gem as a temporary bridge from attr_accessible to strong parameters.

The hop sequence

3.2 → 4.0 → 4.1 → 4.2

Core patterns

Pattern 1: 3.2 → 4.0

Mandatory:

• Ruby 2.0+.
• Switch from attr_accessible / attr_protected to strong parameters (or stay on protected_attributes gem as a bridge).
• New asset pipeline; old Sprockets behavior changed.
• Turbolinks ships (can disable if breaking the app).

attr_accessible removal:

# Rails 3 model
class User < ActiveRecord::Base
  attr_accessible :name, :email
end

# Rails 4 — two paths

# Path A: use the `protected_attributes` gem (keeps old API; quick win for legacy)
gem "protected_attributes_continued"  # community-maintained for 4+

# Path B: migrate to strong parameters (the long-term path)
# Remove attr_accessible from model.
# In controllers:
def user_params
  params.require(:user).permit(:name, :email)
end

Turbolinks:

# Gemfile
gem "turbolinks"

# JS that depends on $(document).ready needs to listen for "page:load" too
$(document).on('page:load ready', function() { ... })

Most Rails 3 apps with custom JS break under Turbolinks. Disable per-link with data-no-turbolink while you migrate.

Pattern 2: 4.0 → 4.1

Spring (preloader) arrives. Configuration changes:

• secrets.yml replaces config/initializers/secret_token.rb.
• Mailer previews (Action Mailer Preview) ship.

Pattern 3: 4.1 → 4.2

Active Job arrives — the abstraction over Sidekiq / DelayedJob / etc. Migrate background jobs:

# Rails 4.1 (Sidekiq worker)
class HardWorker
  include Sidekiq::Worker
  def perform(name); end
end

# Rails 4.2 (Active Job)
class HardWorkerJob < ActiveJob::Base
  queue_as :default
  def perform(name); end
end