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

    scope :open, -> { where.missing(:closure) }
    scope :closed, -> { joins(:closure) }
  end

  def close(user: Current.user)
    create_closure!(user: user)
    track_event "card_closed", user: user
  end

  def reopen
    closure&.destroy!
    track_event "card_reopened"
  end

  def closed?
    closure.present?
  end

  def open?
    !closed?
  end

  def closed_at
    closure&.created_at
  end

  def closed_by
    closure&.user
  end
end

Association concern

# app/models/card/assignable.rb
module Card::Assignable
  extend ActiveSupport::Concern

  included do
    has_many :assignments, dependent: :destroy
    has_many :assignees, through: :assignments, source: :assignee

    scope :assigned_to, ->(users) { joins(:assignments).where(assignments: { assignee: users }).distinct }
    scope :unassigned, -> { where.missing(:assignments) }
  end

  def assign(user)
    assignments.create!(user: user) unless assigned_to?(user)
    track_event "card_assigned", user: user, particulars: { assignee_id: user.id }
  end

  def unassign(user)
    assignments.where(user: user).destroy_all
  end

  def assigned_to?(user)
    assignees.include?(user)
  end
end

Behavior concern with class methods

# app/models/card/searchable.rb
module Card::Searchable
  extend ActiveSupport::Concern

  included do
    scope :search, ->(query) { where("title LIKE ? OR body LIKE ?", "%#{query}%", "%#{query}%") }
  end

  class_methods do
    def search_with_ranking(query)
      search(query).order("search_rank DESC")
    end

    def top_results(query, limit: 10)
      search_with_ranking(query).limit(limit)
    end
  end
end

Controller concern structure

# app/controllers/concerns/card_scoped.rb
module CardScoped
  extend ActiveSupport::Concern

  included do
    before_action :set_card
    before_action :set_board
  end

  private

  def set_card
    @card = Current.account.cards.find(params[:card_id])
  end

  def set_board
    @board = @card.board
  end

  def render_card_replacement
    respond_to do |format|
      format.turbo_stream do
        render turbo_stream: turbo_stream.replace(
          dom_id(@card, :card_container),
          partial: "cards/container",
          locals: { card: @card.reload }
        )
      end
      format.html { redirect_to @card }
    end
  end
end

Naming conventions

• Model concerns (adjectives): Closeable, Publishable, Watchable, Assignable, Searchable, Eventable, Broadcastable, Readable, Positionable
• Controller concerns (nouns): CardScoped, BoardScoped, FilterScoped, CurrentRequest, CurrentTimezone, Authentication

Testing concerns

Test in isolation:

# test/models/concerns/closeable_test.rb
class CloseableTest < ActiveSupport::TestCase
  setup do
    @card = cards(:logo)
  end

  test "close creates closure record" do
    assert_difference -> { Closure.count }, 1 do
      @card.close
    end
    assert @card.closed?
  end

  test "reopen destroys closure record" do
    @card.close
    assert_difference -> { Closure.count }, -1 do
      @card.reopen
    end
    assert @card.open?
  end

  test "closed scope finds closed records" do
    @card.close
    assert_includes Card.closed, @card
    refute_includes Card.open, @card
  end
end

Refactoring workflow

1. Identify the pattern -- Find duplicated code across models/controllers
2. Name the concern -- Use an adjective describing the capability
3. Create the file -- app/models/[model]/[concern].rb or app/controllers/concerns/[concern].rb
4. Move code -- Associations, validations, scopes, methods
5. Include it -- Add include ConcernName to models/controllers
6. Write tests -- Test concern in isolation and in context
7. Remove duplication -- Delete the old code from models/controllers

Files to create

1. Concern file: app/models/card/closeable.rb or app/controllers/concerns/card_scoped.rb
2. Model/Controller: Add include ConcernName
3. Test file: test/models/concerns/closeable_test.rb

See references/concern-catalog.md for the full catalog of concern types.

Boundaries

• Always: Extract repeated code into concerns, keep concerns focused on one aspect, include all related code (associations, scopes, methods), write tests, use extend ActiveSupport::Concern, namespace model concerns under the model
• Ask first: Before creating concerns that span multiple domains, before extracting concerns with complex dependencies, before modifying widely-used concerns
• Never: Create god concerns with too many responsibilities, use concerns to hide service objects, skip included do block for callbacks/associations, create concerns for one-off code