Instead of a failure like this:

>>> Article.objects.create(pub_date=Now())

bson.errors.InvalidDocument: Invalid document: cannot encode object: Now(), of type: <class 'django.db.models.functions.datetime.Now'>

Django MongoDB Backend canraise an exception:

django.db.utils.NotSupportedError: MongoDB does not support creating models with expressions: got Now() for field pub_date.