rails-upgrade-7-to-8

Rails 7 → 8 Upgrade

Upgrade a Rails 7.x app to Rails 8.0. The hop is smaller than 6→7 but has real choices: Solid Queue vs your existing job backend, Propshaft vs Sprockets, the new authentication generator vs Devise. This skill walks the hop with the dual-boot pattern.

Why this matters

Rails 8 doesn't break much code. But the new defaults (Solid Queue, Solid Cache, Propshaft, Kamal 2) are adoptable — and "adopt all at once" is the wrong move. Stage the adoption.

The opinion

Dual-boot with next_rails. Hop 7.x → 7.1 → 7.2 → 8.0 (one minor at a time). Adopt Solid Queue / Cache / Cable only after the version is green — these aren't required by Rails 8, just the default for rails new. Keep Sprockets if you have a complex asset pipeline; migrate to Propshaft as a separate project. Skip the built-in bin/rails generate authentication if you have Devise.

Counter-position: a small app on Rails 7.x can do the upgrade in one PR. Dual-boot is for non-trivial apps where breaking master for a week is unacceptable.

The hop sequence

7.0 → 7.1 → 7.2 → 8.0

Three Rails minor versions. Never skip — rails app:update is per-version diff-aware.

Core patterns

Pattern 1: Set up dual-boot with next_rails

# Gemfile (outside any group)
gem "next_rails", "~> 1.6"

# Existing Rails 7.x:
gem "rails", "~> 7.2.0"

bundle install
bundle exec next --init  # generates Gemfile.next + Gemfile.next.lock

# Gemfile (after init) — next? is true under BUNDLE_GEMFILE=Gemfile.next
gem "rails", next? ? "~> 8.0.0" : "~> 7.2.0"
# ... continue for any gem with version-specific compatibility

Run on the next version:

BUNDLE_GEMFILE=Gemfile.next bin/rspec
BUNDLE_GEMFILE=Gemfile.next bin/rails server

CI runs both Gemfiles in parallel. When the next version is green, swap defaults.

Pattern 2: 7.x → 7.1 hop

Key changes:

• Rails.error.report (the unified error API — see observability-baseline).
• Async query API (load_async).
• composed_of deprecated path; use Attributes API instead.
• ActiveRecord::Base.with_connection (was with_connection).
• Encrypted attributes get easier — encrypts :col is stable.