title: Mongodb cannot use the part (...) to traverse the element ({...: undefined})]
body: <p>After upgrade to 3.0 mongo driver i am receiving some new error on update request. For update like this:</p>
  
  <pre><code>db.table.update({_id: .... } , {$set : { "tags.Tag1" : true }});
  </code></pre>
  
  <p>I am receiving </p>
  
  <pre><code>cannot use the part (tags of tags.Tag1) to traverse the element ({tags: null})]]
  </code></pre>
  
  <p>The problem is that my updated document already contains default value for <code>tags</code> : null. If I manually remove it from document , update starts to work correctly. It is some new behavior for me , and it is happens after updating mongo driver from 2 to 3 ( not even database itself). </p>
  
  <p>But now I wonder now how to avoid this error. I can of course check if "tags" already defined and only then make <code>$set</code> to element or the whole map. But it means 3 requests vs one old and the other problems like atomicity. </p>