Almost entirely off topic, but is "ORM" a fitting term for something that maps documents, and not relations, to objects?
That comes at some cost in MongoDB and document databases in general. Often it is best to start with the opposite of modelling a relational db - start denormalised and normalise where it makes sense to.
The catch with MongoEngine is know that ReferenceFields are dereferenced in the application layer and design your Documents accordingly.
And one of the selling points of NoSQL "document store"-style solutions is that, as they do not use the relational model, your data layout can more closely follow object construction and not need the kind of object-(datastore)-mapping represented by ORM, anyway.