title: Python mongoengine - retrieve _id after saving
body: <p>I need a help in storing and retrieving to mongodb from python using mongoengine. 
  I am relatievely new to mongoengine and i am trying to insert a document using something like below:</p>
  
  <p>Sample code i used for saving:</p>
  
  <pre><code>session = Session()
  session =  session.from_json(sessionjson)
  session.save()
  </code></pre>
  
  <p>It is saving well and good, but is it possible to retrieve _id from the saved document?
  The above save is returning QuerySetManager object but it is not identifying a field called _id.
  Please advice on this </p>