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:

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

   # Multiple models have:
   has_many :comments, as: :commentable
   # Extract to: app/models/concerns/commentable.rb
   

2. Repeated state patterns

   # Multiple models have close/reopen pattern
   # Extract to: Card::Closeable, Board::Publishable, etc.
   

3. Repeated scopes

   # Multiple models have:
   scope :recent, -> { order(created_at: :desc) }
   # Extract to: Timestampable concern
   

4. Repeated controller patterns

   # 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

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

  included do
    has_one :closure, dependent: :destroy