title: How to use $add inside the update() function in Mongo?
body: <p>I'm trying to update this document;</p>
  
  <pre><code>{"dealId" : 201, "commitCount" : 3 }
  </code></pre>
  
  <p>...I just need to increment <code>commitCount</code> by 1, so I'm trying to use the add[] function inside update(), but I can't get it to work...</p>
  
  <pre><code>db.deal.update( {dealId:201},{$set:{commitCount:{$add:['$commitCount',1]} }} )
  </code></pre>
  
  <p>the error I get from Mongo is <code>not okForStorage</code>, no idea what that means, the field does not get updated,  </p>