Pylons gives me an error here when I try to move a comment thread to a different article:

```
Module r2.lib.jsonresponse:181 in _Json        
>>  val = func(self, res, *a, **kw)
Module r2.controllers.validator.validator:78 in newfn        
>>  return fn(self, *a, **kw)
Module r2.controllers.api:229 in POST_move        
>>  currlink._incr('_descendant_karma', -(thing._descendant_karma + thing._ups - thing._downs))
Module r2.lib.db.thing:497 in __getattr__        
>>  return DataThing.__getattr__(self, attr)
Module r2.lib.db.thing:124 in __getattr__        
>>  raise AttributeError, '%s not found' % attr
<type 'exceptions.AttributeError'>: _descendant_karma not found 
```

I had created a couple of articles while on the master branch, then switched to the postmovement branch and tried to move some comments around and got this error. Perhaps it is relying on something that is not being set by current code?