application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  def current_buyer
    if cookies[:buyer_id].present? && (buyer = Buyer.find(cookies[:buyer_id]))
      @current_buyer = buyer
    else
      @current_buyer = Buyer.create
      cookies[:buyer_id] = { value: @current_buyer.id, expires: 1.week.from_now }
    end
    @current_buyer
  end
  helper_method :current_buyer
end

orders_migration.rb

class CreateBuyersAndOrders < ActiveRecord::Migration
  def up
    create_table "buyers", :force => true do |t|
      t.timestamps
    end

    create_table "orders", :force => true do |t|
      t.integer  "buyer_id"
      t.boolean  "completed",  :default => false
      t.datetime "ordered_at"

      t.timestamps
    end

    create_table "order_items", :force => true do |t|
      t.float    "price"
      t.integer  "product_id"
      t.integer  "order_id"
      t.integer  "quantity",   :default => 1

      t.timestamps
    end
  end

  def down
    drop_table :order_items
    drop_table :orders
    drop_table :buyers
  end
end