title: Mongo DB: how to select items with nested array count > 0
body: <p>The database is near 5GB. I have documents like:</p>
  
  <pre><code>{
      _id: ..
      user: "a"
      hobbies: [{
          _id: ..
          name: football
      },
      {
          _id: ..
          name: beer
      }
      ...
      ]
  }
  </code></pre>
  
  <p>I want to return users who have more then 0 "hobbies"
  I've tried </p>
  
  <pre><code>db.collection.find({"hobbies" : { &amp;gt : 0}}).limit(10)
  </code></pre>
  
  <p>and it takes all RAM and no result.</p>
  
  <ol>
  <li>How to do conduct this select? </li>
  <li>And how to return only: id, name, count ?</li>
  <li>How to do it with c# official driver?</li>
  </ol>
  
  <p>TIA</p>
  
  <p>P.S.
  <a href="https://stackoverflow.com/questions/7456033/how-to-get-embedded-arrays-size-in-mongo">near</a> i've found:
  "Add new field to hande category size. It's a usual practice in mongo world."
  is this true?</p>