def formfield(self, **kwargs):
        kwargs.setdefault('form_class', TreeNodeChoiceField)
        return super(TreeForeignKey, self).formfield(**kwargs)
Adds items from extra_context dict to context.  If a value in extra_context
dataset/ETHPy150Open django-mptt/django-mptt/mptt/fields.py/TreeForeignKey.formfield--crossover with--dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.2/django/views/generic/create_update.py/apply_extra_context