Minimal example that causes an error when deep copying a mongoengine Document.

deepcopy.py

from copy import deepcopy
from mongoengine import Document, ReferenceField, connect

class Foo(Document):
    pass

class Bar(Document):
    foo = ReferenceField(Foo)

connect('tmp-deepcopy')

Foo.drop_collection()
foo = Foo()
foo.save()

Bar.drop_collection()
bar = Bar(foo=foo)
bar.save()

deepcopy(bar) # succeeds
bar = Bar.objects.get(id=bar.id)
deepcopy(bar) # fails

output.txt

Traceback (most recent call last):
  File "deepcopy.py", line 22, in <module>
    deepcopy(Bar.objects.get(id=bar.id))
  File "/usr/lib/python2.6/copy.py", line 189, in deepcopy
    y = _reconstruct(x, rv, 1, memo)
  File "/usr/lib/python2.6/copy.py", line 338, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib/python2.6/copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.6/copy.py", line 255, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.6/copy.py", line 162, in deepcopy
    y = copier(x, memo)
  File "/usr/lib/python2.6/copy.py", line 255, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib/python2.6/copy.py", line 171, in deepcopy
    copier = getattr(x, "__deepcopy__", None)
  File "build/bdist.linux-i686/egg/bson/dbref.py", line 81, in __getattr__
KeyError: '__deepcopy__'