rails-upgrade-4-to-5

Rails 4 → 5 Upgrade

Rails 4.2 → 5.0 is one of the biggest API breaks in the past decade. params stop being a Hash. Strong parameters become mandatory. ApplicationRecord arrives. Treat as a multi-month project for a non-trivial app.

The opinion

Dual-boot with next_rails. Hop 4.2 → 5.0 → 5.1 → 5.2. Ruby 2.2.2+ for Rails 5.0; Ruby 2.5+ for 5.2. Budget weeks-to-months for a non-trivial app.

The hop sequence

4.2 → 5.0 → 5.1 → 5.2

Core patterns

Pattern 1: 4.2 → 5.0 — the breaking changes

Required code changes:

# Rails 4 — base model inheriting from ActiveRecord::Base
class User < ActiveRecord::Base
end

# Rails 5 — inherit from ApplicationRecord
class User < ApplicationRecord
end

# Generate the new base class
bin/rails generate application_record

params change — biggest gotcha:

# Rails 4 — params was a HashWithIndifferentAccess
params[:user][:email]                 # worked
params.permit(:foo, :bar)             # explicit but not required
User.create(params[:user])            # silently allowed any column

# Rails 5 — params is ActionController::Parameters
params[:user][:email]                 # works (subclass of Hash)
params.merge(...)                     # returns Parameters, not Hash
User.create(params[:user])            # raises ForbiddenAttributesError — must permit

Audit every Model.create(params[:foo]) / update(params[:foo]) call. Add permit.

Other 5.0 changes:

• ActionCable arrives (no impact unless you use it).
• belongs_to is required by default (optional: true to opt out).
• Test framework default switches; Rails.test may need --clean runs.
• head :ok after a respond_to block is no longer needed (slight behavior change).

Pattern 2: 5.0 → 5.1

• yarn arrives for managing JS packages.
• System tests (Capybara wrapper) ship.
• form_for / form_tag start to be deprecated in favor of form_with.
• secrets.yml partially deprecated (full deprecation in 5.2).