I have been going back and forth over which Python API to use when interacting with Mongo. I did a quick survey of the landscape and identified three leading candidates.
The official Mongodb documentation talks about djongo. It works by translating SQL queries into mongodb queries.
You don't need django-nonrel to run it.
All native Django contrib modules (eg. admin, user, session) work without any modification.
MongoEngine requires rewriting contrib modules and last I checked, the native admin module didn't run on MongoEngine.
Your existing models run without any ORM translation as well.