Newer versions of Django explicitly prevent managers being
        accessed from abstract classes, which is behaviour the search
        API has always relied on. Here we reinstate it.

def contribute_to_class(self, model, name):
        """
        Newer versions of Django explicitly prevent managers being
        accessed from abstract classes, which is behaviour the search
        API has always relied on. Here we reinstate it.
        """
        super(SearchableManager, self).contribute_to_class(model, name)
        setattr(model, name, ManagerDescriptor(self))