Minimal example that causes a mysterious uniqueness constraint when inserting into mongoengine.

output.txt

Traceback (most recent call last):
  File "unique.py", line 15, in <module>
    B(foo=True).save()
  File "/usr/local/lib/python2.6/dist-packages/mongoengine-0.4-py2.6.egg/mongoengine/document.py", line 85, in save
mongoengine.queryset.OperationError: Tried to save duplicate unique keys (E11000 duplicate key errorindex: tmp-unique.a.$foo_1  dup key: { : null })

unique.py

from mongoengine import *

class A(Document):
    pass

class B(A):
    foo = BooleanField(unique=True)

connect('tmp-unique')
A.drop_collection()
B.drop_collection()

A().save()
A().save()
B(foo=True).save()