title: Rails Ancestry Gem: how to write scope to match all parents
body: <p>Using the Rails <code>ancestry</code> gem ,
  what is the best way to write a scope on the User model to find all records which have children / which are parents?</p>
  
  <pre><code>class User &lt; ActiveRecords::Base
    has_ancestry
  
     def is_manager?
       has_children
     end
  
     scope :is_manager, -&gt; { ... ? ... }
  
  end
  </code></pre>