Can't seem to create an association

I have two tables, user & provider and I'm struggling to set up the association between them.

A simplified table structure for the two is:

CREATE TABLE users (     id integer not null,     login string not null,     password string not null )

CREATE TABLE providers (     id integer not null,     user_id integer not null,     ... )

and I've used the generater to create two modules, as follows (cut down)

class User < ActiveRecord::Base   has_one :provider

  def invoices_raised     provider.get_invoices   end



class Provider < ActiveRecord::Base   has_many :invoices   belongs_to :user

  def get_invoices     invoices.find(:all)   end end

I have a controller that calls invoices_raised :

class AccountController < ApplicationController   model :user

  before_filter :get_invoices

  def get_invoices     @user = User.find(@session['user'].id)

    @invoices_raised = @user.invoices_raised   end



but inside the invoices_raised method I get the following error:

You have a nil object when you didn't expect it! The error occured while evaluating nil.get_invoices

I thought that using belongs_to & has_one was supposed to implicitly create a provider instance for me?