Assign model to session variable in rails 4

I would like to know if it is possible to save an article object in a variable of session for example I have this code but it gives me this error:

  

undefined method 'add_cesta' for "#": String

This is what I'm trying to do:

This is my driver :

class TiendaController < ApplicationController


  def index
    @titulo = "Bienvenido a la Tienda"
    @articulos = Articulo.all.order("nombre").page(params[:page]).per_page(4)
  end

  def quienes_somos
    @titulo = "Bienvenido a la Tienda"      
  end

  def contacto
    @titulo = "Bienvenido a la Tienda"
  end

  def anadir_producto
      @articulo = Articulo.find(params[:id])
      @carro = sesion_carrito
      @carro.add_cesta(@articulo)
      flash[:info] ="Producto añadido #{@articulo.nombre}"      
      redirect_to inicio_url
  end

  def ver_carro
    if tenemos_sesion?
      @carro = session[:carro]
    else
      flash[:info] = "Donde Vas! Compra antes"
      redirect_to inicio_url
    end
  end

  def vaciar_carrito
      session[:carro] = nil
      flash[:info] = "Carrito vacio"
      redirect_to inicio_url
  end

  private

  def tenemos_sesion?
    session[:carro]
  end

  def sesion_carrito      
      session[:carro] ||= Carro.new
  end
end

this is my class carro.rb within models:

class Carro
  attr_reader :cesta

  def initialize
    @cesta = []
  end

  def add_cesta(articulo)
    @cesta << articulo    
  end

end

Theoretically it should work, I have not got something that tells me that you can not but also something that supports you, someone can tell me if it is possible and if I can I'm doing wrong.