Rails Engine Author

Use this skill when the task is to create, scaffold, or refactor a Rails engine.

Favor maintainability over cleverness. A good engine has a narrow purpose, a clear host-app integration story, and a small public API.

Keep this skill focused on structure and design. Use adjacent skills for installer details, deep test coverage, release workflow, or documentation work.

Quick Reference

HARD-GATE

Before engine work is complete, confirm all of the following:

1. The root file is minimal: requires version, configuration, and engine only.
2. Public-facing engines use isolate_namespace.
3. The root module exposes .configure yielding a Configuration object.
4. Host model references stay configurable strings (for example "User"), never ::User.
5. Engine code never auto-applies migrations at boot.
6. The host contract is documented: what host configures, what engine exposes.

Pitfalls

Workflow

1. Identify the engine type before writing code. Scaffold with the correct generator:

   rails plugin new my_engine --mountable   # mountable engine
   rails plugin new my_engine --full        # full engine (non-isolated)
   rails plugin new my_engine               # plain Railtie/gem

2. Define the host-app contract (what host must provide; what engine exposes).
3. Create the minimal engine structure. Checkpoint: bundle exec rake inside the engine must pass.
4. Implement features behind the namespace. Checkpoint: mount engine in dummy app routes and verify with bundle exec rails routes.
5. Plan and write minimum integration coverage through the dummy app.
6. Document the host-app contract clearly enough for follow-on work.

If the user does not specify the engine type, infer it from the requested behavior and say which type you chose.

Recommended Structure

Use a structure close to this:

my_engine/
  lib/
    my_engine.rb
    my_engine/version.rb
    my_engine/engine.rb
    generators/
  app/
    controllers/
    models/
    jobs/
    views/
  config/
    routes.rb
    locales/
  db/
    migrate/
  spec/ or test/
    dummy/

Keep the root module small:

• lib/my_engine.rb: requires version, engine, and public configuration entrypoints.
• lib/my_engine/engine.rb: engine class, initializers, autoload/eager-load behavior, asset/config hooks.
• lib/my_engine/version.rb: version only.

Host App Contract

Before implementation, define:

• What the host app must add: mount route, initializer, migrations, credentials, background jobs, or assets.
• What the engine exposes: models, controllers, helpers, configuration, rake tasks, generators, middleware, or events.
• Which extension points are supported: config block, adapter interface, callbacks, or service objects.

Prefer one explicit configuration surface, for example:

MyEngine.configure do |config|
  config.user_class = "User"
  config.audit_events = true
end

Do not scatter configuration across unrelated constants and initializers.

Implementation Rules

• Use isolate_namespace for mountable and public-facing engines.
• Keep initializers idempotent and safe in development reloads; use config.to_prepare only for reload-sensitive code (e.g. decorators).
• Treat migrations as host-owned — provide install/copy generators, never apply silently. Do NOT use config.paths['db/migrate'] or ActiveRecord::Migrator in initializers:

# WRONG — auto-applies migrations at boot, host loses control
initializer 'my_engine.migrations' do
  config.paths['db/migrate'] << root.join('db/migrate')
end

# RIGHT — host copies via install generator, runs manually
# See rails-engine-installers for generator patterns

• Reference host app models through configurable class names or adapters; do not hard-code host constants.
• Expose integration seams through services, adapters, or hooks — not direct host constants.
• Keep assets and generators namespaced and idempotent.
• Put reusable domain logic in POROs/services, not hooks.

Use rails-engine-installers for generator-heavy setup, rails-engine-testing for dummy-app coverage, and rails-engine-reviewer for audits.

Testing Expectations

Minimum coverage through the dummy app (not just isolated classes):

• Engine integration tests through the mounted dummy app.
• Routing/request tests for all mountable engine endpoints.
• Configuration tests for each supported host customization option.
• Generator tests when install/setup generators exist.

Examples

Minimal root module:

# lib/my_engine.rb
require "my_engine/version"
require "my_engine/configuration"
require "my_engine/engine"

module MyEngine
  class << self
    def configuration
      @configuration ||= Configuration.new
    end

    def configure
      yield(configuration)
    end
  end
end

Minimal mountable engine class:

# lib/my_engine/engine.rb
module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine

    config.generators do |g|
      g.test_framework :rspec
      g.fixture_replacement :factory_bot
    end
  end
end

Routes namespaced under engine:

# config/routes.rb
MyEngine::Engine.routes.draw do
  root to: 'dashboard#index'
  resources :widgets, only: %i[index show]
end

Verification

Before calling the engine structure done:

1. Check lib/my_engine.rb is still requires + configuration only.
2. Search engine files for hard-coded host constants such as ::User or ::Employee.
3. Search engine boot code for migration auto-apply patterns such as db:migrate, ActiveRecord::Migrator, or config.paths['db/migrate'].
4. Mount the engine in the dummy app and verify routes load correctly.
5. Confirm the host contract doc names required host config and engine-provided surfaces.

Output Style

When asked to create or refactor an engine:

1. State the engine type you are using and why.
2. Show the target file structure.
3. Implement the smallest viable set of files first.
4. Add tests before broadening features.
5. Call out assumptions about the host app explicitly.

Optional Reference Pattern

If a real-world engine corpus is available, inspect comparable engines before making structural decisions. Prefer matching successful patterns from mature engines over inventing new conventions.

For a reusable starter layout and file stubs, read reference.md.

Integration