title: Mongoengine : How to update specific fields of an existing document?
body: <p>I have an existing mongo document which has been exposed over a REST API. The API request will contain certain fields from the document which either needs to be updated with new values or insert new values in them if the field is null. How to perform the update on fields of an existing mongoengine document? I'm using marshmallow-mongoengine for serialization on flask.</p>
  
  <p>The problem that I'm facing is that if a certain field is missing in the request payload, on calling update with the remaining fields as kwargs leads to setting the missing fields as None. How can update or insert only the fields given in the payload?</p>