Rails Engine Testing

Use this skill when the task is to create or improve test coverage for a Rails engine.

Prefer integration confidence over isolated test quantity. The main goal is to prove the engine behaves correctly inside a host app.

Quick Reference

HARD-GATE

EVERY engine MUST have a dummy app for testing.

Generate one if it doesn't exist:

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.

Testing Order

1. Identify the engine type and public behaviors.
2. Decide which behaviors need unit tests versus dummy-app integration tests.
3. 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 rather than adding more specs on top of a broken foundation.
4. Add request, routing, configuration, and generator coverage as needed.
5. Add regression tests for coupling or reload bugs before refactoring.
6. Verify: dummy app exercises real host integration; routes tested through engine namespace; configurable seams covered with at least one non-default case; generators safe to run twice.

Minimum Baseline

For a non-trivial engine, aim for:

• one dummy-app boot or integration spec
• one request or routing spec for mounted endpoints
• one configuration spec for host customization
• unit tests for public services or POROs

If generators exist, add generator specs. If decorators or reload hooks exist, add reload-focused coverage.

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

For generator and reload-safety spec examples, see EXAMPLES.md.

Pitfalls

Integration