# Concern Patterns

Creates and refactors model and controller concerns for shared behavior.

- Repository: https://github.com/ThibautBaissac/rails_ai_agents (671 stars)
- Category: Engineering
- License: MIT
- Last updated: 2026-06-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add ThibautBaissac/rails_ai_agents --skill concern-patterns -g -a claude-code -y`
Codex: `npx skills add ThibautBaissac/rails_ai_agents --skill concern-patterns -g -a codex -y`
Cursor: `npx skills add ThibautBaissac/rails_ai_agents --skill concern-patterns -g -a cursor -y`
Gemini CLI: `npx skills add ThibautBaissac/rails_ai_agents --skill concern-patterns -g -a gemini-cli -y`

## Description

Creates and refactors model and controller concerns for shared behavior. Use when extracting shared code, organizing models with horizontal concerns, DRYing up controllers, or when user mentions concerns, mixins, or modules. WHEN NOT: Logic used by only one model (keep in place), service object extraction (use model-patterns), or job organization (use job-patterns).

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# Concern Patterns (37signals)

Concerns for horizontal behavior, inheritance for vertical specialization.

## Project knowledge

**Tech Stack:** Rails 8.2 (edge), ActiveSupport::Concern
**Location:** `app/models/[model]/` for model concerns, `app/controllers/concerns/` for controller concerns

**Commands:**
```bash
ls app/models/concerns/                      # List shared concerns
ls app/models/card/                          # List Card concerns
bin/rails runner "puts Card.included_modules" # Check usage
bin/rails test test/models/                  # Run model tests
```

## Core principles

Each concern should be:
- **Self-contained:** All related code (associations, validations, scopes, methods) in one place
- **Cohesive:** Focused on one aspect (e.g., `Closeable`, `Watchable`, `Searchable`)
- **Composable:** Models include multiple concerns to build up behavior

## When to extract a concern

### Extract when you see:

1. **Repeated associations across models**
   ```ruby
   # Multiple models have:
   has_many :comments, as: :commentable
   # Extract to: app/models/concerns/commentable.rb
   ```

2. **Repeated state patterns**
   ```ruby
   # Multiple models have close/reopen pattern
   # Extract to: Card::Closeable, Board::Publishable, etc.
   ```

3. **Repeated scopes**
   ```ruby
   # Multiple models have:
   scope :recent, -> { order(created_at: :desc) }
   # Extract to: Timestampable concern
   ```

4. **Repeated controller patterns**
   ```ruby
   # Multiple controllers load parent resource
   # Extract to: ParentScoped concern
   ```

### Do NOT extract when:
- Code is used by only one model (YAGNI)
- You'd create a god concern with unrelated methods
- Logic should be in explicit model methods instead

## Model concern structure

### State management concern

```ruby
# app/models/card/closeable.rb
module Card::Closeable
  extend ActiveSupport::Concern

  included do
    has_one :closure, dependent: :destroy

The rest: https://github.com/ThibautBaissac/rails_ai_agents/blob/HEAD/.claude_37signals/skills/concern-patterns/SKILL.md