class ApplicationController < ActionController::Base
 protect_from_forgery with: :exception
 before_action :configure_permitted_parameters, if: :devise_controller?
 protected
 def configure_permitted_parameters
    devise_parameter_sanitizer.for(:sign_up)<<[:first_name,:last_name,:terms_and_conditions]
 end
 private
  def current_cart
   Cart.find(session[:cart_id])
   rescue ActiveRecord::RecordNotFound
   cart = Cart.create
   session[:cart_id] = cart.id
   cart
  end
end

RecordNotFound:

It the exception class defined in the

~/.rvm/gems/ruby-version/gems/activerecord-version/lib/active_record/errors.rb

This RecordNotFound Exception is thrown when Active Record cannot find record by given id or set ids.

RecordNotFound class is inherited from ActiveRecordError which is inherited from StandardError class which is subclass of Exception class.Thus Exception class is the super class of all the classes.

If we are using the rescue statement we need to use find() in such a way tha it raises an exception , that is , passing the id you want to find.

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

Cart.find method will find the cart_id temporarily present in the session , if the cart _id is present in the session the current_cart method simply returns the whole object of the cart otherwise it will throw an RecordNotFound Exception which means that the cart_id is not present in the session , then it will create a cart and  store it in the session.Lastly it returns the object of the cart.