title: How to do "insert if not exist else update" with mongoengine?
body: <p>I'm working with mongoengine in Django,<br>
  this is my document defination:  </p>
  
  <pre><code>class Location(mongoengine.Document):  
      user_id = mongoengine.IntField(required=True)  
      point = mongoengine.GeoPointField(required=True)
  </code></pre>
  
  <p>I want to do this:<br>
  given a <code>user_id</code> and a <code>point</code>:<br>
  if there is no document that have this <code>user_id</code>, create one with the <code>user_id</code> and <code>point</code> and save it;<br>
  else update the document with <code>user_id</code> with <code>point</code>.<br>
  Can I do this in one statement with mongoengine?</p>