title: Count query for nested document in MongoDB
body: <p>I want to count how many Items are available in the sub-document <code>Recommendations</code> in the following document.</p>
  
  <pre><code>{
     catgoryId: 13,
     Recommendations:
     {
         {ItemId: 135, value= 0.8},
         {ItemId: 136, value= 0.7},
         ....
     }
  }
  </code></pre>
  
  <p>}</p>