application_controller.rb

class ApplicationController < ActionController::Base
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception

  private
  
  def current_cart
    @current_cart ||= begin
      Cart.find(session[:cart_id])
    rescue ActiveRecord::RecordNotFound
      cart = Cart.create
      session[:cart_id] = cart.id
      cart
    end
  end
  
  helper_method :current_cart
end

class StoreController < ApplicationController
  def index
    @products = Product.all
    @cart = current_cart
  end
end