title: Querying a repeated property by count in NDB
body: <p>Is there an efficient mechanism for querying by the number of items in a repeated property in NDB?</p>
  
  <p>I'd like to do something like:</p>
  
  <pre><code>Class.query(class.repeated_property.count == 2)
  </code></pre>
  
  <p>but of course this doesn't work.</p>