Strategy + Factory + Null Object Calculator Pattern

One API for the client: Calculator::Factory.for(entity).calculate. The factory picks the strategy; NullService handles unknown variants safely.

HARD-GATE: Tests Gate Implementation

For each component (Factory → BaseService → NullService → Concrete):

1. Write the spec — contexts per variant, plus the NullService path
2. Run it — verify it fails because the component does not exist yet
3. Implement the component — minimum code to make the spec pass
4. Run again — confirm green, then proceed to the next component

Quick Reference

File Structure

app/services/<calculator_name>/
├── factory.rb
├── base_service.rb
├── null_service.rb
├── standard_service.rb
├── premium_service.rb

1. Factory

# frozen_string_literal: true

module PricingCalculator
  class Factory
    SERVICE_MAP = {
      'standard' => StandardPricingService,
      'premium'  => PremiumPricingService
    }.freeze

    def self.for(order)
      plan = order.plan
      return NullService.new(order) unless plan&.active?

      service_class = SERVICE_MAP[plan.name] || NullService
      service_class.new(order)
    end
  end
end

No qualifying context or unknown variant → NullService. For full BaseService and NullService implementations, see IMPLEMENTATION.md.

2. Usage

price = PricingCalculator::Factory.for(order).calculate

Single entry point rule: Factory.for(entity) is the only permitted access path. Clients never instantiate service classes directly. If you see StandardPricingService.new(order) outside of Factory, that is a bug — route through the factory.

3. Tests (RSpec)

Factory dispatch (all branches):

RSpec.describe PricingCalculator::Factory do
  describe '.for' do
    it 'returns NullService when plan is nil' do
      order = create(:order, plan: nil)
      expect(described_class.for(order)).to be_a(PricingCalculator::NullService)
    end

    it 'returns StandardPricingService for standard plan' do
      order = create(:order, plan: create(:plan, name: 'standard', active: true))
      expect(described_class.for(order)).to be_a(PricingCalculator::StandardPricingService)
    end
  end
end

Cover inactive plan, each variant, and unknown variant. See TESTING.md for NullService and concrete service specs.

Pitfalls

Integration