Refactoring using OOP

checkout_service.rb

# CheckoutService takes a cart, and checkouts each cart items
class CheckoutService
  def initialize(cart, params)
    @cart   = cart
    @params = params
  end
  
  def run
      create_order
      if order_approved? && cart.checked_out_at.nil?
        create_credits
        lift_reservation_restrictions
        fulfill_items
        checkout_cart
        deliver_order_success_email if order_completed?
      end
   end
    
    def create_credits
      CreditAllocationService.call(@cart)
    end
  
    def lift_reservation_restrictions
      Checkout::LiftReservationRestrictionService.call(@cart)
    end
end

# CreditAllocationService goes through each cart items, and creates credit for user
# Hidden object here is 'packages'
class CreditAllocationService
  def initialize(cart)
    @cart = cart
    @user = @cart.user
  end

  def call
    packages.each do |package|
      # TODO: Remove number_of_sessions after products
      next if package.number_of_sessions == 0

      @user.credits << credit_for_package(package)
    end

    @user.save!
  end

  def self.call(cart)
    new(cart).call
  end

  private

  def credit_for_package(package)
    @user.credits.new do |credit|
      credit.status             = :active
      credit.cart_item_id       = package.cart_item_id
      credit.count              = package.number_of_credits
      credit.credit_type        = package.credit_type
      credit.expires_at         = package.expires_at
      credit.number_of_sessions = package.number_of_credits
      credit.unlimited          = package.unlimited?
      credit.unlimited_validity = package.unlimited_validity?
      credit.daily_limit        = package.daily_limit
    end
  end

  def packages
    @cart.cart_items.creditable.includes(:purchasable).map do |item|
      PackageDecorator.new(item)
    end
  end
end
 
# LiftReservationRestrictionService goes through each reservation_restriction and updates it to paid
class Checkout::LiftReservationRestrictionService
  def self.call(cart)
    cart.reservation_restrictions.update_all(paid_at: Time.now)
  end
end

# Goal of the refactor is to find a common interface for 
# packages and reservation restriction
# therefore we introduce #fulfill as the common interface 

# Refactored version
class CheckoutService
  attr_reader :params, :cart, :order

  def initialize(cart, params)
    @cart   = cart
    @params = params
  end

  def run
    create_order
    if order_approved? && cart.checked_out_at.nil?
      fulfill_items
      checkout_cart
      deliver_order_success_email if order_completed?
    end
  end

  private
  
  def fulfill_items
    cart.cart_items.each do |item|
      fulfillment_klass_for(item).new(item).fulfill
    end
  end

  def fulfillment_klass_for(item)
    "Fulfillment::#{item.purchasable.class}".constantize
  end
end

class Fulfillment::Package
  def initialize(cart_item)
    @cart_item = cart_item
    @package   = cart_item.package
    @user      = cart_item.user
  end

  def fulfill
    # TODO: Remove number_of_sessions after products
    return if @package.number_of_sessions == 0

    @user.credits.create do |credit|
      credit.status             = :active
      credit.cart_item_id       = @cart_item.id
      credit.count              = number_of_credits
      credit.credit_type        = @package.credit_type
      credit.expires_at         = expires_at
      credit.number_of_sessions = number_of_credits
      credit.unlimited          = @package.unlimited?
      credit.unlimited_validity = @package.unlimited_validity?
      credit.daily_limit        = @package.daily_limit
    end
  end

  private

  def number_of_credits
    return if @package.unlimited?

    @package.number_of_sessions * @cart_item.quantity
  end

  def expires_at
    return if @package.unlimited_validity?

    Time.current + (@package.validity_period_days.days * @cart_item.quantity)
  end
end 

class Fulfillment::ReservationRestriction
  def initialize(cart_item)
    @reservation_restriction = cart_item.purchasable
  end

  def fulfill
    @reservation_restriction.update(paid_at: Time.now)
  end
end