Basic Shopping Cart implementation, using ActiveRecord for persistence of Order/Item and a module to encapsulate the logic of a Cart

cart.rb

module MyApp
  class Cart
    attr_accessor :order, :items # <== ActiveRecord Instance of Order & Item

    def initialize(user, token)
      @order ||= user.blank? ? Order.load_for_guest(token) : Order.load_for_customer(user)
      @items = @order.items
    end
  
    def add(product_id, quantity=1)
      if @items.where(product_id: product_id).size.zero?
        @items.create(product_id: product_id, quantity: quantity)
      else
        @items.where(product_id: product_id).first.increment(:quantity, quantity)
      end
    end
    
    def add_multiple(params) # params = { [{quantity:<val>, product_id:<val>}, ...] }
      params.each do |item|
        add(item[:product_id], item[:quantity])
      end
    end
    
    def remove(product)
      @items.where(product: product).map(&:destroy)
    end
    
    def clean
      @items.destroy_all
    end
    
    # ...
  end
end

carts_controller.rb

class CartsController < ApplicationController
  before_filter :load_cart
  
  # params = { items: [ 
  #                     {quantity: 2, product_id: 613}, 
  #                     {quantity: 1, product_id: 3271}, 
  #                     {quantity: 4, product_id: 981}  
  #                   ] 
  #          }
  def update
    @cart.add_multiple(params[:items])
  end
  

  private

  def load_cart
    @cart ||= MyApp::Cart.new(current_user, session[:cart_id])
    session[:cart_id] ||= @cart.id
  end

end

item.rb

class Item < ActiveRecord::Base
  belongs_to :product
  
  after_initialize :set_defaults
  
  def set_defaults
    self[:quantity] ||= 1
  end
  
end

order.rb

class Order < ActiveRecord::Base
  belongs_to :user
  has_many :items

  # .... State Machine conf ....

  scope :pending, where(state: 'pending')

  def self.load_for_customer(user)
    self.pending.where(user: user).last.blank? ? self.create(user: user) : self.pending.where(user: user).last
  end

  def self.load_for_guest(cart_id)
    if (cart_id.blank? || self.pending.find_by_id(cart_id).blank?)
      self.create
    else
      self.pending.find_by_id(cart_id)
    end
  end
end