title: $addToSet from my python code replace data instead of appending it
body: <p>I'm trying to save data in my collection using Pymongo. </p>
  
  <pre><code>port = 27017
  conn = pymongo.MongoClient('localhost', port)
  db   = conn.dbcoll
  newReservationId ="Value change from front-end"
  customerData = { 'firstName':"Jamesss", 'lastName':"Jamesss" }
  db.person.update({'_id':'1223'},  customerData )
  db.person.update({'_id':'1223'},  {"$addToSet":{"reservationList":newReservationId }} ) 
  conn.close()
  </code></pre>
  
  <p>Every time the field "reservationList" get overwrite by the new value "newReservationId". It is suppose to append the new value "newReservationId" in the array.</p>
  
  <p>Can anyone please let me know why  $addToSet doesn't work the way it is suppose to ?</p>
  
  <p>Thanks for your help.</p>
  
  <p>Cheers,</p>