gistfile1.txt

>>> from mongoengine import *
>>> class FacebookUser(Document):
...     meta = { "allow_inheritance": False }
...     facebook_id = StringField(primary_key = True, max_length = 20)
...     name = StringField()
...     def __unicode__(self):
...         return u"/".join((self.facebook_id, self.name or u""))
... 
>>> connect("test")
Connection('localhost', 27017)
>>> FacebookUser.objects.delete()
>>> FacebookUser.objects
[]
>>> FacebookUser.objects.get_or_create(facebook_id = "1234")
(<FacebookUser: 1234/>, True)
>>> FacebookUser.objects
[]
>>> FacebookUser.objects.get_or_create(facebook_id = "1234", name = "John Smith")
(<FacebookUser: 1234/John Smith>, True)
>>> FacebookUser.objects
[<FacebookUser: 1234/John Smith>]
>>> FacebookUser.objects.get_or_create(facebook_id = "5678", name = "David Peters")
(<FacebookUser: 5678/David Peters>, True)
>>> FacebookUser.objects
[<FacebookUser: 1234/John Smith>, <FacebookUser: 5678/David Peters>]
>>> FacebookUser.objects.get_or_create(facebook_id = "1234", name = "Bjarne Stroustrup")
(<FacebookUser: 1234/Bjarne Stroustrup>, True)
>>> FacebookUser.objects
[<FacebookUser: 5678/David Peters>, <FacebookUser: 1234/Bjarne Stroustrup>]

When I add extra attributes, such as the name (line 18), then the object is properly saved.