title: How to convert pandas series element into string
body: <p>I have a pandas dataframe in which I have mongoDB's objectIDs stored. I want to fetch new documents from mongoDB, for this I am using last document ObjectId stored in pandas dataframe. But when I query the database it gives me an error.</p>
  
  <p>Here is what I am doing. </p>
  
  <pre><code>df = pd.DataFrame(list(db.dataset2.find()))
  
  last_inserted_id = (df['_id'].tail(1))
  
  for i in db.dataset2.find({"_id": {"$gt": ObjectId(last_inserted_id) }}):
       print i
  </code></pre>
  
  <p>Then it gives me an error..</p>
  
  <pre><code>TypeError: id must be an instance of (str, unicode, ObjectId), not &lt;class      
  'pandas.core.series.Series'&gt;
  </code></pre>
  
  <p>This is how my dataframe looks like..</p>
  
  <pre><code>   _id  \
  0  5663110c691db01e0cac9d89   
  1  56631205691db01e0cac9d8a   
  
                     dish                                order_area order_id  
  0  [{u'dish_substitute': u'Yes', u'home_chef_name...        NaN  order_1   
  1  [{u'dish_substitute': u'Yes', u'home_chef_name...        NaN  order_2   
  
   order_lat     order_long              order_time user_id  
  0  73.955741   40.772027 2015-12-05 16:30:04.345  user_1  
  1  43.955741   70.772027 2015-12-05 16:34:13.281  user_2
  </code></pre>
  
  <p>How to deal with this?<br>
  please help..</p>