def apply_delta(self, delta):
        """Apply delta to our state and return a copy of the
        affected object as it was before and after the update, e.g.:

            old_obj, new_obj = self.apply_delta(delta)

        old_obj may be None if the delta is for the creation of a new object,
        e.g. a new application or unit is deployed.

        new_obj will never be None, but may be dead (new_obj.dead == True)
        if the object was deleted as a result of the delta being applied.

        """
        history = (
            self.state
            .setdefault(delta.entity, {})
            .setdefault(delta.get_id(), collections.deque())
        )

        history.append(delta.data)
        if delta.type == 'remove':
            history.append(None)

        entity = self.get_entity(delta.entity, delta.get_id())
        return entity.previous(), entity
Apply delta to our state and return a copy of the
        affected object as it was before and after the update, e.g.:

            old_obj, new_obj = self.apply_delta(delta)

        old_obj may be None if the delta is for the creation of a new object,
        e.g. a new application or unit is deployed.

        new_obj will never be None, but may be dead (new_obj.dead == True)
        if the object was deleted as a result of the delta being applied.