title: Rails - query for where has parent
body: <p>I have this simple association</p>
  
  <pre><code>class User
      has_many :items
  
  class Item
      belongs_to :item
  </code></pre>
  
  <p>I'm not sure how to write a query to find all Items that has/hasnt a user.</p>
  
  <p>I tried this but user isn't a column I can query on the Item</p>
  
  <pre><code>Item.where(:user =&gt; nil) 
  </code></pre>
  
  <p>Thanks!</p>