>>> class A(db.Document):
...   meta = {'allow_inheritance': True}
... 
>>> class B(db.Document):
...   cross_ref = db.ReferenceField(A, reverse_delete_rule=db.NULLIFY)
... 
>>> class C(A):
...   cross_ref = db.ReferenceField(B, reverse_delete_rule=db.NULLIFY)
... 
>>> b = B()
>>> b.save()
<B: B object>
>>> c = C()
>>> c.save()
<C: C object>
>>> b.cross_ref = c
>>> c.cross_ref = b
>>> b.save()
<B: B object>
>>> c.save()
<C: C object>
>>> b.id
ObjectId('56b24a45efdbe273c79e44d6')
>>> c.id
ObjectId('56b24a50efdbe273c79e44d7')
>>> [(_.id, _.cross_ref.id) for _ in B.objects]
[(ObjectId('56b24a45efdbe273c79e44d6'), ObjectId('56b24a50efdbe273c79e44d7'))]
>>> [(_.id, _.cross_ref.id) for _ in C.objects]
[(ObjectId('56b24a50efdbe273c79e44d7'), ObjectId('56b24a45efdbe273c79e44d6'))]
>>> c.delete()
>>> B.objects.count()
1
>>> C.objects.count()
0
>>> [(_.id, _.cross_ref.id) for _ in B.objects]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
AttributeError: 'NoneType' object has no attribute 'id'
>>> [_.id for _ in B.objects]
[ObjectId('56b24a45efdbe273c79e44d6')]
>>> [_.id for _ in C.objects]
[]
>>>