title: Rails scope for null or empty?
body: <p>I want to scope all the records where <code>cancelled_at</code> IS NULL or empty/blank.</p>
  
  <p>I've found lots of posts asking for IS NOT NULL/empty/blank...but I need the opposite.</p>
  
  <p>I'm sure it's painfully simple, but I'm stumped.</p>
  
  <p>Here's what I've currently got:</p>
  
  <pre><code>scope :active, where("cancelled_at IS NULL")
  </code></pre>