Test Engine

HARD-GATE

EVERY engine MUST have a dummy app for testing.
If it doesn't exist, generate it:
cd my_engine && bundle exec rails plugin new . --dummy-path=spec/dummy --skip-git

Validate the dummy app boots before proceeding:
cd spec/dummy && bundle exec rails runner "puts 'Boot OK'"

If this fails, check the engine's `engine.rb` initializer order and ensure the engine is correctly mounted in `spec/dummy/config/routes.rb` before writing any specs.

Core Process

1. Identify the engine type and public behaviors.
2. Add the smallest integration test that proves mounting and boot work. Verify it passes before continuing — if it fails, check engine.rb initializer order and mount configuration.
3. Add request, routing, configuration, and generator coverage as needed.
4. Add regression tests for coupling bugs before refactoring.
5. Run the full test suite (bundle exec rspec) to verify all specs pass.

Minimal request spec to prove the engine mounts:

# spec/requests/my_engine/root_spec.rb
require 'rails_helper'

RSpec.describe 'MyEngine mount', type: :request do
  it 'returns ok for the engine root' do
    get my_engine.root_path
    expect(response).to have_http_status(:ok)
  end
end

Configuration spec (engine respects host config):

# spec/my_engine/configuration_spec.rb
RSpec.describe MyEngine::Configuration do
  around do |example|
    original = MyEngine.config.widget_count
    MyEngine.config.widget_count = 3
    example.run
    MyEngine.config.widget_count = original
  end

  it 'uses configured value' do
    expect(MyEngine.config.widget_count).to eq(3)
  end
end

Advanced Coverage Patterns

Namespace-scoped routing spec:

# spec/routing/my_engine/widgets_routing_spec.rb
require 'rails_helper'

RSpec.describe MyEngine::WidgetsController, type: :routing do
  routes { MyEngine::Engine.routes }

  it 'routes GET /widgets to widgets#index' do
    expect(get: '/widgets').to route_to('my_engine/widgets#index')
  end
end

Generator idempotency spec:

# spec/generators/my_engine/install_generator_spec.rb
require 'rails_helper'
require 'generators/my_engine/install/install_generator'

RSpec.describe MyEngine::Generators::InstallGenerator, type: :generator do
  destination File.expand_path('../tmp', __dir__)

  before { prepare_destination }

  it 'generates the initializer' do
    run_generator
    expect(file('config/initializers/my_engine.rb')).to exist
  end

  it 'is idempotent on re-run' do
    run_generator
    expect { run_generator }.not_to raise_error
    expect(file('config/initializers/my_engine.rb')).to exist
  end
end

Output Style

When completing engine test setup, summarise results as:

Engine Test Report — [Engine Name]
- Dummy app: [location], boot ✓, migrations ✓
- Specs: mounting ✓, generators ✓, core (<n> examples, 0 failures)
- Suite: bundle exec rspec — <n> examples, 0 failures

Integration

Skill	When to chain
create-engine	When structuring the engine for testability or adding configuration seams
review-engine	When validating test coverage adequacy or identifying gaps
write-tests	When improving spec structure, matchers, or shared examples