Create Engine Installer

Use this skill when the task is to design or review how a host app installs and configures a Rails engine — generating initializers, copying migrations, mounting routes, or exposing a single install command.

Quick Reference

HARD-GATE

Validation Workflow WHEN building or reviewing an install generator:

1. GENERATE:  Run the generator against a clean host app
2. VERIFY:    Check output files exist in the correct host paths
3. RERUN:     Run the generator a second time
4. CONFIRM:   No duplicate files, routes, or initializer blocks inserted
5. DOCUMENT:  List what was generated vs. what the user must do manually
6. TEST:      Cover both single-run and rerun behavior in generator specs

DO NOT ship a generator without completing steps 3 and 4.

Core Process

1. Ensure setup is explicit, repeatable, and safe to rerun.
2. Configure only in initializers (avoid boot-time mutation).
3. Guard operations with File.exist? or Thor's inject_into_file with a marker to ensure idempotency.
4. Copy migrations to host db/migrate; let the host run them.
5. Document rollback steps and required env vars.
6. Ensure install docs match generator behavior exactly.

Idempotency guards — check before creating or injecting:

def create_initializer
  return if File.exist?(File.join(destination_root, 'config/initializers/my_engine.rb'))
  create_file 'config/initializers/my_engine.rb', <<~RUBY
    MyEngine.configure do |config|
      config.user_class = "User"
    end
  RUBY
end

def mount_route
  # inject_into_file with force: false skips insertion if sentinel already present
  inject_into_file 'config/routes.rb',
    "\n  mount MyEngine::Engine, at: '/admin'\n",
    after: "Rails.application.routes.draw do",
    force: false
end

Minimal rerun spec (must always pass):

it 'does not duplicate the route mount on rerun' do
  2.times { run_generator }
  expect(File.read(file('config/routes.rb')).scan('mount MyEngine::Engine').size).to eq(1)
end

Extended Resources

Generator Checklist

• Files created in correct host paths

• No duplicate inserts on rerun (validated manually and in specs)

• Sensible defaults that are easy to edit

• Clear output telling the user what remains manual

• Rollback steps documented

• Install docs match what the generator actually produces

• EXAMPLES.md (full generator class and complete spec suite)

• assets/README.md

Output Style

1. Use idiomatic Rails Thor generator commands.
2. Provide clear, minimal, idempotent generator code.
3. Output clear terminal instructions for the user.
4. Language — Must be in English unless explicitly requested otherwise.

Integration