assigned linked models can't be removed

[evrenesat]

In [21]: w.personel
Out[21]: <Personel: foo 2323 (1234 | None)>

In [22]: w.personel = None

In [23]: w.save()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-23-9bf4ce6bbe5a> in <module>()
----> 1 w.save()

/home/evren/.virtualenvs/ulakbus/lib/python2.7/site-packages/pyoko/model.pyc in save(self)
    585
    586     def save(self):
--> 587         self.objects.save_model(self)
    588         for i in range(len(self.new_back_links)):
    589             if self.new_back_links:

/home/evren/.virtualenvs/ulakbus/lib/python2.7/site-packages/pyoko/db/base.pyc in save_model(self, model)
    218         if model:
    219             self.model = model
--> 220         clean_value = self.model.clean_value()
    221         if not self.model.is_in_db():
    222             self.model.key = None

/home/evren/.virtualenvs/ulakbus/lib/python2.7/site-packages/pyoko/model.pyc in clean_value(self)
    448         self._clean_field_value(dct)
    449         self._clean_node_value(dct)
--> 450         self._clean_linked_model_value(dct)
    451         return dct
    452

/home/evren/.virtualenvs/ulakbus/lib/python2.7/site-packages/pyoko/model.pyc in _clean_linked_model_value(self, dct)
    433         # get vales of linked models
    434         for name in self._linked_models:
--> 435             dct[un_camel_id(name)] = getattr(self, name).key
    436
    437     def clean_field_values(self):

AttributeError: 'NoneType' object has no attribute 'key'