Rails Concern Generator (TDD)

Creates concerns (ActiveSupport::Concern modules) for shared behavior with specs first.

Quick Start

• Write failing spec testing the concern behavior

• Run spec to confirm RED

• Implement concern in app/models/concerns/ or app/controllers/concerns/

• Run spec to confirm GREEN

When to Use Concerns

Good use cases:

• Shared validations across multiple models

• Common scopes used by several models

• Shared callbacks (e.g., UUID generation)

• Controller authentication/authorization helpers

• Pagination or filtering logic

Avoid concerns when:

• Logic is only used in one place (YAGNI)

• Creating "god" concerns with unrelated methods

• Logic should be a service object instead

TDD Workflow

Step 1: Create Concern Spec (RED)

For Model Concerns, test via a model that includes it:

spec/models/concerns/[concern_name]_spec.rb

RSpec.describe [ConcernName] do

Create a test class that includes the concern

let(:test_class) do Class.new(ApplicationRecord) do self.table_name = "events" # Use existing table include [ConcernName] end end

let(:instance) { test_class.new }

describe "included behavior" do it "adds the expected methods" do expect(instance).to respond_to(:method_from_concern) end end

describe "#method_from_concern" do it "behaves as expected" do expect(instance.method_from_concern).to eq(expected_value) end end

describe "class methods" do it "adds scope" do expect(test_class).to respond_to(:scope_name) end end end

Alternative: Test through an actual model that uses the concern:

spec/models/event_spec.rb

RSpec.describe Event, type: :model do describe "[ConcernName] behavior" do describe "#method_from_concern" do let(:event) { build(:event) }

  it "does something" do
    expect(event.method_from_concern).to eq(expected)
  end
end

end end

For Controller Concerns, test via request specs:

spec/requests/[feature]_spec.rb

RSpec.describe "[Feature]", type: :request do describe "pagination (from Paginatable concern)" do let(:user) { create(:user) } before { sign_in user }

it "paginates results" do
  create_list(:resource, 30, account: user.account)
  get resources_path
  expect(response.body).to include("page")
end

end end

Step 2: Run Spec (Confirm RED)

bundle exec rspec spec/models/concerns/[concern_name]_spec.rb

OR

bundle exec rspec spec/models/[model]_spec.rb

Step 3: Implement Concern (GREEN)

Model Concern:

app/models/concerns/[concern_name].rb

module [ConcernName] extend ActiveSupport::Concern

included do # Callbacks before_validation :generate_uuid, on: :create

# Validations
validates :uuid, presence: true, uniqueness: true

# Scopes
scope :with_uuid, ->(uuid) { where(uuid: uuid) }
scope :recent, -> { order(created_at: :desc) }

end

Class methods

class_methods do def find_by_uuid!(uuid) find_by!(uuid: uuid) end end

Instance methods

def generate_uuid self.uuid ||= SecureRandom.uuid end

def short_uuid uuid&.split("-")&.first end end

Controller Concern:

app/controllers/concerns/[concern_name].rb

module [ConcernName] extend ActiveSupport::Concern

included do before_action :set_locale helper_method :current_locale end

class_methods do def skip_locale_for(*actions) skip_before_action :set_locale, only: actions end end

private

def set_locale I18n.locale = params[:locale] || I18n.default_locale end

def current_locale I18n.locale end end

Step 4: Run Spec (Confirm GREEN)

bundle exec rspec spec/models/concerns/[concern_name]_spec.rb

Common Concern Patterns

Pattern 1: UUID Generation

app/models/concerns/has_uuid.rb

module HasUuid extend ActiveSupport::Concern

included do before_validation :generate_uuid, on: :create validates :uuid, presence: true, uniqueness: true end

private

def generate_uuid self.uuid ||= SecureRandom.uuid end end

Pattern 2: Soft Delete

app/models/concerns/soft_deletable.rb

module SoftDeletable extend ActiveSupport::Concern

included do scope :active, -> { where(deleted_at: nil) } scope :deleted, -> { where.not(deleted_at: nil) }

default_scope { active }

end

def soft_delete update(deleted_at: Time.current) end

def restore update(deleted_at: nil) end

def deleted? deleted_at.present? end end

Pattern 3: Searchable

app/models/concerns/searchable.rb

module Searchable extend ActiveSupport::Concern

class_methods do def search(query) return all if query.blank?

  where("name ILIKE :q OR email ILIKE :q", q: "%#{query}%")
end

end end

Pattern 4: Auditable

app/models/concerns/auditable.rb

module Auditable extend ActiveSupport::Concern

included do has_many :audit_logs, as: :auditable, dependent: :destroy

after_create :log_creation
after_update :log_update

end

private

def log_creation audit_logs.create(action: "created", changes: attributes) end

def log_update return unless saved_changes.any? audit_logs.create(action: "updated", changes: saved_changes) end end

Pattern 5: Controller Filterable

app/controllers/concerns/filterable.rb

module Filterable extend ActiveSupport::Concern

private

def apply_filters(scope, allowed_filters) allowed_filters.each do |filter| if params[filter].present? scope = scope.where(filter => params[filter]) end end scope end end

Usage

In Models:

class Event < ApplicationRecord include HasUuid include SoftDeletable include Searchable end

In Controllers:

class ApplicationController < ActionController::Base include Filterable end

Checklist

• Spec written first (RED)

• Uses extend ActiveSupport::Concern

• included block for callbacks/validations/scopes

• class_methods block for class-level methods

• Instance methods outside blocks

• Single responsibility (one purpose per concern)

• Well-named (describes what it adds)

• All specs GREEN