Rails Engine Extraction

Use this skill when the task is to move existing code out of a Rails app and into an engine.

Prefer incremental extraction over big-bang rewrites. Preserve behavior first, then improve design.

HARD-GATE

DO NOT extract and change behavior in the same step. Extraction must preserve existing behavior; refactoring and improvements belong in a separate step after the move is complete and verified.

Extraction Order

1. Identify the bounded feature to extract — one coherent responsibility.
2. List hard dependencies on the host app (models, services, config).
3. Define the future engine boundary and host contract.
4. Move stable domain logic first: POROs, services, value objects, policies, query objects. Delay direct host model references, authentication, route ownership, and asset integration.
5. Add adapters or configuration seams for host-owned dependencies. Replace hardcoded host references with config values, adapter objects, or service interfaces.
6. Move controllers, routes, views, or jobs only after seams are clear.
7. Keep regression coverage green throughout each slice.

Each slice must have: one coherent responsibility, minimal new public API, passing regression tests, and a clear next step.

Pitfalls

Examples

First slice (move PORO, no host model yet):

# Move the file into the engine and adjust the namespace
mkdir -p my_engine/app/services/my_engine
mv app/services/pricing/calculator.rb my_engine/app/services/my_engine/pricing_calculator.rb

# Before (in host app): module Pricing; class Calculator
# After (in engine):
module MyEngine
  class PricingCalculator
    def initialize(line_items)
      @line_items = line_items
    end

    def total
      @line_items.sum { |item| item.price * item.quantity }
    end
  end
end

Verify regression coverage still passes before proceeding to the next slice:

bundle exec rspec spec/services/pricing/ spec/requests/orders/

Move engine-local models in the same slice, or keep host models and inject via an adapter in a later slice.

Adapter for host dependency:

# lib/my_engine/configuration.rb
module MyEngine
  class Configuration
    attr_accessor :current_user_provider
  end

  def self.config
    @config ||= Configuration.new
  end

  def self.configure
    yield config
  end
end

# In engine: resolve user through config, not hardcoded constant
class OrderCreator
  def initialize(user)
    @user = user
  end

  def self.for_request(request)
    new(MyEngine.config.current_user_provider.call(request))
  end
end

# Host initializer (config/initializers/my_engine.rb)
MyEngine.configure do |config|
  config.current_user_provider = ->(request) { request.env['warden'].user }
end

Integration