title: Writing "not in" sql query using AREL
body: <p>I have a scope defined as follows:</p>
  
  <pre><code>scope :ignore_unavailable, lambda { 
    where([ "Item.id NOT IN (SELECT id FROM Cars WHERE Cars.status = 'NA'" ])
  }
  </code></pre>
  
  <p>Currently its using hardcoded tables names. How can I improve it using frameworks like Arel ?
  Will appreciate any help here.</p>
  
  <p>I am on Rails 3.2</p>