guest.js

function Guest(session) {
  this.session = session;
  this.name = "Anonymous";
  this.avatar = session.avatar || "https://api.adorable.io/avatars/128/" + Math.random();
  session.avatar = this.avatar;
}

Guest.prototype.getCart = function() {
  this.cart = this.cart || Cart.find(this.session.cartId) || Cart.create();
  return cart;
};

Guest.prototype.setCart = function(cart) {
  this.cart = cart;
}

Guest.prototype.save = function(cart) {
  this.session.cartId = cart && cart.id;
  return true;
}

guest.rb

class Guest
  def initialize(session)
    self.session = session
  end

  def name
    "Anonymous"
  end

  def avatar
    session[:avatar] ||= "https://api.adorable.io/avatars/128/#{rand}"
  end

  def cart
    @cart ||= Cart.find_by(id: session[:cart_id]) || Cart.create
  end

  def cart=(cart)
    @cart = cart
  end

  def save
    session[:cart_id] = cart && cart.id
    true
  end

  def persisted?
    false
  end

  private

    attr_accessor :session
end

I've made some changes to reproduce a little better the ActiveRecord::Bases behavior, only "saving" the data, meaning writing to the session, when the save function is called.