[Answered ]-Getting "database error" (using django, djangotoolbox, mongodbengine of Django-nonrel)

Django-nonrel does not support many-to-may relationships.

two alternative to work around this are.

First you can use ListFields by importing it like so:
from djangotoolbox.fields import ListField

The ListField stores a list of IDs.
Note however that the number of entities are limited to 1MB (I think) but this very adequate if you are not storing huge volumes/entity.

Another alternative is to use another entity to map the many to many relationships.
This will eliminate the above limitation but there will be an overhead in storing and querying the mapping entities.