Meaningful reference errors

[thomasst]

It's been painful trying to figure out which references are broken.

This PR needs more testing, but what do you think? (Feel free to take this PR over)

In [7]: c.pk
Out[7]: 'acti_JYLfSSgIrpHESqDfNZJAzi5eiw9F3q5zqJS7xtFZG3y'

In [8]: c._db_data['transferred_to']
Out[8]: 'acti_nonexistent'

In [9]: c.transferred_to.number
---------------------------------------------------------------------------
DoesNotExist                              Traceback (most recent call last)
/home/closeio/closeio/closeio/exporter/documents/__init__.py in <module>()
----> 1 c.transferred_to.number

/home/closeio/venv/src/mongoengine/build/lib/mongoengine/base/proxy.py in __getattr__(self, name)
     63         if name == '__members__':
     64             return dir(self._get_current_object())
---> 65         return getattr(self._get_current_object(), name)
     66 
     67     def __setitem__(self, key, value):

/home/closeio/venv/src/mongoengine/build/lib/mongoengine/base/proxy.py in _get_current_object(self)
    209                     )
    210                 error += ') has been deleted.'
--> 211                 raise DoesNotExist(error)
    212             document = self.__document_type._from_son(son)
    213             object.__setattr__(self, '_DocumentProxy__document', document)

DoesNotExist: Document (activity acti_nonexistent, referenced by Call acti_JYLfSSgIrpHESqDfNZJAzi5eiw9F3q5zqJS7xtFZG3y in field transferred_to) has been deleted.

[tsx]

Didn't try it myself but looks reasonable.

[tsx]

Can we also add the extra data as exception attributes? At some point we would want to inspect the parent object and parsing error message is not a good idea.

[wojcikstefan]

If we do this, we should start with doing it in the upstream version so that we have a consistent interface. A more informative message on the same exception is good. Raising what would essentially amount to a different exception is not good.

[tsx]

I'd suggest to avoid holding onto the parent instance as this might cause memory leaks. Store just the class (name or reference), and the instance pk.

[wojcikstefan]

This is pretty cool, I'm willing to give it a shot. I agree with V that ideally we'd just store the class name/reference and the ID of the parent instance.