Ruby on Rails

Hi all i am following this like for basic paypal http://railscasts.com/episodes/141-paypal-basics
and i got this error


View:

<%= link_to "Checkout", @cart.paypal_url() %>

undefined method `paypal_url' for nil:NilClass
 despite i have this code in my model called Card

Cart Model:

class Cart < ActiveRecord::Base
  
  
  
  

def paypal_url(return_url)
  values = {
    :business => '',
    :cmd => '_cart',
    :upload => 1,
    :return => return_url,
    :invoice => id
  }
  line_items.each_with_index do |item, index|
    values.merge!({
      "amount_#{index+1}" => 500,
      "item_name_#{index+1}" => car,
      "item_number_#{index+1}" => 5,
      "quantity_#{index+1}" => 1
    })
  end
  "https://www.sandbox.paypal.com/cgi-bin/webscr?" + values.to_query
end


end
 
it is unable to identify it 

what is the problem


Cheers

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