title: Mongo query for number of items in a sub collection
body: <p>This seems like it should be very simple but I can't get it to work.  I want to select all documents A where there are one or more B elements in a sub collection.</p>
  
  <p>Like if a Store document had a collection of Employees.  I just want to find Stores with 1 or more Employees in it.</p>
  
  <p>I tried something like:</p>
  
  <pre><code>{Store.Employees:{$size:{$ne:0}}}
  </code></pre>
  
  <p>or</p>
  
  <pre><code>{Store.Employees:{$size:{$gt:0}}}
  </code></pre>
  
  <p>Just can't get it to work.</p>