Hello! Old thread, but I recently made my MongoDB Mathematica driver open-source here: https://github.com/zbjornson/MongoDBLink.
Here is a related SE post: mathematica.stackexchange.com/questions/40731
There is a good presentation with practical code examples about Mathematica with MongoDB here:
MongoDB with WL- .NB Presentation
Wolfram Language 11.3 now has support for interfacing with MongoDB: http://reference.wolfram.com/language/MongoLink/guide/MongoLinkOperations.html
That's cool! Thanks for sharing this new feature.