PayRoll application, embedded in Rails, borrowing from Use Case Driven Architecture and DCI

your_application.rb

## PayRoll gem

# lib/pay_roll.rb
module PayRoll
  class << self
    attr_accessor :employee_directory
      def config
        yield self
      end
    end
  end
end

# lib/pay_roll/services/pay_day_service.rb
class PayRoll::PayDayService
  def initialize(date=Date.now)
    @date = date

    @employees = PayRoll.employee_directory.active
    @employees.each { |e| e.extend(Payable) }
  end

  def execute
    @employees.each do |e|
      if e.pay_date?(@date)
        pc = PayCheck.new(e.calculate_pay(@date))
        e.send_pay(pc)
      end
    end
  end
end

# lib/pay_roll/roles/payable.rb
module PayRoll::Payable
  def pay_date?(date)
    # ...
  end

  def send_pay(pay_check)
    # ...
  end
end

## Rails application

# app/controllers/pay_day_controller.rb
# Yes, this would make more sense to be run in a scheduled job, but wanted to show
# an example of services used in a Rails controlle
class PayDayController < ApplicationController
  def create
    PayRoll::PayDayService.new.execute
    redirect_to :back, :notice => "Pay day has been successfully completed"
  end
end

# config/initializers/pay_roll.rb
PayRoll.config do |config|
  employee_directory = Employee
end

# models/employee.rb
class Employee < ActiveRecord::Base
  scope :active, where(:active => true)
end