Reference to Abstract Document generates an InvalidDocument

Hi,

Regarding #1155, when I query on a object with a instance variable that has a ReferenceField to an abstract document and this field is already populated with one concrete subclass object.

I get an error when Mongoengine tries to run the method "from_son".
The error I get is the following

Invalid data to create a `User` instance.
notifications - type object 'UserNotifications' has no attribute 'id'  

Which makes sense because UserNotifications is indeed abstract. The reference field has the ObjectID of a EditorNotifications object.

Here's the entire stack trace in case it helps:

 Traceback (most recent call last):
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/core/handlers/base.py", line 132, in get_response
        response = wrapped_callback(request, *callback_args, **callback_kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/views/generic/base.py", line 71, in view
        return self.dispatch(request, *args, **kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/utils/decorators.py", line 34, in _wrapper
        return bound_func(*args, **kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
        return view_func(request, *args, **kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/utils/decorators.py", line 30, in bound_func
        return func.__get__(self, type(self))(*args2, **kwargs2)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/utils/decorators.py", line 34, in _wrapper
        return bound_func(*args, **kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/utils/decorators.py", line 110, in _wrapped_view
        response = view_func(request, *args, **kwargs)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/utils/decorators.py", line 30, in bound_func
        return func.__get__(self, type(self))(*args2, **kwargs2)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/views/generic/base.py", line 89, in dispatch
        return handler(request, *args, **kwargs)
      File "/Users/AfonsoGraca/Unbabel/unbabel/payment/views.py", line 68, in post
        if self.form.is_valid():
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/forms/forms.py", line 184, in is_valid
        return self.is_bound and not self.errors
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/forms/forms.py", line 176, in errors
        self.full_clean()
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/forms/forms.py", line 393, in full_clean
        self._clean_form()
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/django/forms/forms.py", line 417, in _clean_form
        cleaned_data = self.clean()
      File "/Users/AfonsoGraca/Unbabel/unbabel/payment/forms.py", line 22, in clean
        last_cash_out = self.editor.get_last_cash_out()
      File "/Users/AfonsoGraca/Unbabel/unbabel/users/mongo_models.py", line 1855, in get_last_cash_out
        return self.get_transactions().filter(type='Cash Out').first()
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 264, in first
        result = queryset[0]
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 161, in __getitem__
        return queryset._document._from_son(queryset._cursor[key],
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 1481, in _cursor
        self._cursor_obj = self._collection.find(self._query,
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 1515, in _query
        self._mongo_query = self._query_obj.to_query(self._document)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 89, in to_query
        query = self.accept(SimplificationVisitor())
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 139, in accept
        return visitor.visit_combination(self)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 39, in visit_combination
        return Q(**self._query_conjunction(queries))
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/queryset/visitor.py", line 59, in _query_conjunction
        combined_query.update(copy.deepcopy(query))
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 163, in deepcopy
        y = copier(x, memo)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 257, in _deepcopy_dict
        y[deepcopy(key, memo)] = deepcopy(value, memo)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 190, in deepcopy
        y = _reconstruct(x, rv, 1, memo)
      File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/copy.py", line 336, in _reconstruct
        y.__setstate__(state)
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/base/document.py", line 200, in __setstate__
        data["_data"] = self.__class__._from_son(data["_data"])._data
      File "/Users/AfonsoGraca/.virtualenvs/unbabel/lib/python2.7/site-packages/mongoengine/base/document.py", line 725, in _from_son
        raise InvalidDocumentError(msg)
    InvalidDocumentError: Invalid data to create a `User` instance.
    notifications - type object 'UserNotifications' has no attribute 'id'

Cheers

Start at the _from_son entry point named in the traceback and inspect how a ReferenceField targeting an abstract document resolves a stored ObjectID for a concrete subclass. Reproduce the case with an abstract UserNotifications reference populated by an EditorNotifications object; done means deserialization no longer raises InvalidDocumentError.