title: Rails scope testing for nil
body: <p>I have an expenses table.  It has a column invoice_id.</p>
  
  <p>I would like 2 scopes - one for billed expenses and one for not billed.
  I thought I could just test to see if invoice_id was nil.</p>
  
  <pre><code>scope :notbilled, where(:invoice_id == nil)
  scope :billed, where(:invoice_id != nil)
  </code></pre>
  
  <p>But, that doesn't work.</p>
  
  <p>Any ideas?</p>
  
  <p>Thanks!</p>