Mongo Documents with ObjectId is not being updated and create new document with Mongo.ObjectId

Hi,
My Mongodb collection contains document with ObjectId. When I get the collection in client side, all my ObjectId is converted to string by default. Thus making me difficult to perform operation like update from client side. I can run the update query from server side which works fine but not in offline mode.
The insert command creates a string _id in document while I want Mongo.ObjectId as _id and the same happens for updating the document. The document with string _id gets updated while the documents with ObjectId doesn’t.

Can you please explain.. Really need a fix for this.