# Extract Engine

> Use when extracting a host-app feature into a Rails engine. Do not change behavior in the same step. Trigger words: extract to engine, move feature, host coupling.

- Skill: `igmarin/extract-engine` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add igmarin/extract-engine`
- Raw SKILL.md: https://api.skillmd.com/api/skills/igmarin/extract-engine/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: igmarin (https://skillmd.com/u/igmarin)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/igmarin/extract-engine

---


# Extract Engine

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.

## Quick Reference

| Phase | Focus |
|-------|-------|
| Prep | Identify bounded feature and host dependencies |
| Logic | Move stable domain logic (POROs, services) first |
| Seams | Add adapters/config for host dependencies |
| Web | Move controllers, routes, views last |

## HARD-GATE

```text
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.
```

## Core Process

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.

## Extended Resources

**Pitfalls**

| Pitfall | What to do |
|---------|------------|
| Extracting too much at once | One bounded slice per step |
| Direct host references in engine | Use adapters or config |
| Behavior changes mixed with extraction | Preserve behavior first; refactor after |
| Circular dependencies introduced | Verify import graph before each slice |
| Implicit host contract | Explicitly document and test the host app contract |

**Examples**

**First slice (move PORO, no host model yet):**
```bash
mkdir -p my_engine/app/services/my_engine
mv app/services/pricing/calculator.rb my_engine/app/services/my_engine/pricing_calculator.rb
```
```ruby
# 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:
```bash
bundle exec rspec spec/services/pricing/ spec/requests/orders/
```

**Adapter for host dependency:**
```ruby
module MyEngine
  def self.current_user_for(request)
    config.current_user_provider.call(request)
  end
end

# usage
OrderCreator.for_request(request) # resolves via MyEngine.current_user_for(request)
```

## Output Style

1. Propose one small, bounded extraction slice at a time.
2. Outline the files moving, the new boundaries, and the regression tests to run.
3. Language — Must be in English unless explicitly requested otherwise.

## Integration

| Skill | When to chain |
|-------|----------------|
| create-engine | Engine structure, host contract, namespace design after extraction |
| test-engine | Dummy app, regression tests, integration verification |
| refactor-code | Behavior-preserving refactors before or after extraction slices |