dual-boot

When invoked via /dual-boot, follow the setup workflow in workflows/setup-workflow.md to set up dual-boot for this project. If the user provides version arguments (e.g. /dual-boot 7.0 7.1), use them as the current and target versions. If no arguments are provided, detect the current version from the Gemfile and ask the user for the target version.

Dual-Boot Skill

• Purpose: Set up and manage dual-boot environments for Rails, Ruby, or core Gemfile dependencies using the next_rails gem
• Key Gem: next_rails
• Methodology: Based on FastRuby.io upgrade best practices and "The Complete Guide to Upgrade Rails" ebook
• Attribution: Content based on "The Complete Guide to Upgrade Rails" by FastRuby.io (OmbuLabs)

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Overview

Dual-booting allows a Ruby application to maintain two sets of dependencies simultaneously. While most commonly used for Rails version upgrades, the same approach works for upgrading Ruby versions or any core dependency in your Gemfile (e.g., sidekiq, devise, pg).

This skill helps you:

• Set up dual-boot with the next_rails gem
• Configure the Gemfile to support two versions of Rails, Ruby, or other key dependencies
• Branch application code using NextRails.next?
• Configure CI to test against both dependency sets
• Clean up dual-boot code after the upgrade is complete

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CRITICAL: Always Use NextRails.next? — Never Use Feature Detection

When code would break on one version and needs a different implementation on the other, always use NextRails.next? from the next_rails gem to branch — never feature detection. Do not use respond_to?, defined?, const_defined?, or any other feature-detection pattern for version branching.

Why feature detection is problematic:

• Hard to understand: readers must know which version introduced a method or constant to grasp the intent
• Hard to maintain: respond_to? / defined? checks pile up and become impossible to clean up because their purpose is lost
• Fragile: may give wrong results if gems monkey-patch methods or constants in or out
• Obscures intent: the code says "does this exist?" when it means "are we on the next Rails version?"

Why NextRails.next? is better:

• Explicit and readable: anyone reading the code immediately understands "this branch is for the next version"
• Easy to clean up: after the upgrade, search for NextRails.next? and remove all branches
• The standard: established dual-boot mechanism in the FastRuby.io methodology

Pattern

Reach for NextRails.next? only when the old and new APIs are genuinely two-sided — the old one raises on the next version and the new one doesn't exist on the current version. A plain deprecation warning is not a reason to branch: if the new API works on both sides, migrate the call site directly.

❌ WRONG — Do NOT use feature detection (respond_to?, defined?, etc.):

test_request =
  if ActionController::TestRequest.respond_to?(:create)
    ActionController::TestRequest.create
  else
    ActionController::TestRequest.new
  end

✅ CORRECT — Use NextRails.next?:

test_request =
  if NextRails.next?
    ActionController::TestRequest.create
  else
    ActionController::TestRequest.new
  end

This is a genuine two-sided case (Rails 4.2 → 5.0). On 4.2, ActionController::TestRequest.new takes an optional env; on 5.0 new requires two non-optional arguments so the 4.2 call fails. Rails 5.0 introduces TestRequest.create, which does not exist on 4.2 — so each side raises on the other.

Put the next-version branch on top so cleanup is mechanical: after the upgrade, keep the if body and drop the else. See references/code-patterns.md for more examples.

When to Apply

Use NextRails.next? branching for:

• Removed methods or constants where the replacement only exists on the new side (e.g., a gem-provided method replaced by a native Rails method with different syntax)
• Incompatible signature or return-type changes where the old and new forms genuinely fail on opposite sides
• Gem version differences where the gem's API is different across versions pinned to each Rails version
• Initializer changes where a config or middleware raises on one side
• Ruby version differences (e.g., syntax changes, stdlib removals)

Not a reason to branch: plain deprecation warnings. If the new API works on both versions (e.g. config.fixture_path= → config.fixture_paths=, update_attributes → update before its removal), migrate the call site directly — do not wrap it in NextRails.next?. See references/code-patterns.md "When NOT to Branch: Deprecations".

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Trigger Patterns

Claude should activate this skill when user says:

• "Set up dual boot for my Rails app"
• "Help me dual-boot Rails [x] and [y]"
• "Dual-boot Ruby [x] and [y]"
• "Set up dual boot for upgrading [dependency]"
• "Add next_rails to my project"
• "Configure dual-boot CI"
• "Clean up dual-boot code"
• "Remove next_rails after upgrade"
• "How do I use NextRails.next?"
• "Set up Gemfile for two Rails versions"
• "Set up Gemfile for two Ruby versions"

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Core Workflows

Workflow 1: Set Up Dual-Boot

See workflows/setup-workflow.md for the complete step-by-step process.

Summary: 0. Verify deprecation warnings are not silenced (see references/deprecation-tracking.md)

1. Check if dual-boot is already set up (look for Gemfile.next)

2. Add next_rails gem to Gemfile

3. Run bundle install

4. Run next_rails --init (only if Gemfile.next does NOT exist)

5. Configure Gemfile with next? conditionals for the dependency being upgraded

6. Install dependencies for both versions:

   • Current: bundle install
   • Next: cp Gemfile.lock Gemfile.next.lock && BUNDLE_GEMFILE=Gemfile.next bundle install

7. Verify both versions work

Workflow 2: Add Version-Dependent Code

When proposing code changes that need to work with both dependency sets:

1. Verify next_rails is set up (check for Gemfile.next)
2. Use NextRails.next? for branching — never respond_to?
3. Keep the next? branch (new version code) on top
4. Keep the else branch (old version code) below

See references/code-patterns.md for examples.

Workflow 3: Configure CI

See references/ci-configuration.md for CI setup with GitHub Actions, CircleCI, and Jenkins.

Workflow 4: Clean Up After Upgrade

See workflows/cleanup-workflow.md for the complete post-upgrade cleanup process.

Summary:

1. Search for all NextRails.next? references
2. Keep only the NextRails.next? (true) branch code
3. Remove all else branches
4. Remove next? method from Gemfile
5. Remove next_rails gem if no longer needed
6. Remove Gemfile.next and replace Gemfile.lock with Gemfile.next.lock
7. Update CI to remove dual-boot configuration
8. Run full test suite
9. Commit cleanup changes

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Available Resources

Core Documentation

• SKILL.md - This file (entry point)

Reference Materials

• references/deprecation-tracking.md - Detecting silenced deprecations and configuring tracking (Rails 3.0+)
• references/code-patterns.md - NextRails.next? usage examples in application code
• references/ci-configuration.md - CI setup for dual-boot (GitHub Actions, CircleCI, Jenkins)
• references/gemfile-examples.md - Gemfile configuration patterns for dual-boot

Workflows

• workflows/setup-workflow.md - Step-by-step dual-boot setup
• workflows/cleanup-workflow.md - Post-upgrade dual-boot removal

Examples

• examples/basic-setup.md - Basic dual-boot setup example

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Key Principles

1. Ensure deprecation warnings are visible — silenced deprecations mean you can't track upgrade progress
2. Never run next_rails --init if Gemfile.next exists — it will duplicate the next? method
3. Always use NextRails.next? for version-dependent code — never respond_to?
4. Test both versions using the project's own test runner. Detect it before running anything: check the Gemfile for rspec-rails, parallel_tests, turbo_tests, or minitest-parallel_fork, check whether the app has spec/ or test/, and prefer wrappers like bin/test when they exist. Every bundle exec rspec in the workflows, examples, and CI reference is illustrative; substitute the detected command (e.g. bin/rails test, bundle exec parallel_rspec spec/, bin/test) and prefix with BUNDLE_GEMFILE=Gemfile.next (or the next CLI) to run it against the next dependency set.
5. Clean up after upgrade — search for and remove all NextRails.next? branches
6. Add next_rails at the Gemfile root level — not inside a :development or :test group

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Quick Reference

Command	Purpose
next_rails --init	Initialize dual-boot (creates Gemfile.next symlink)
next bundle install	Install gems for the next dependency set
next bundle exec rspec	Run tests with the next dependency set
BUNDLE_GEMFILE=Gemfile.next bundle exec rails server	Start server with next version
BUNDLE_GEMFILE=Gemfile.next bundle exec rspec	Alternative to next command

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See CHANGELOG.md for version history and current version.