rails-concern

Rails Concern Generator (TDD)

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

Quick Start

1. Write failing spec testing the concern behavior
2. Run spec to confirm RED
3. Implement concern in app/models/concerns/ or app/controllers/concerns/
4. 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: