'Install a listener on the two m2m relations.'

def _initialize_signal_car(self, add_default_parts_before_set_signal=False):

    models.signals.m2m_changed.connect(self.m2m_changed_signal_receiver, Car.optional_parts.through)
    if add_default_parts_before_set_signal:
        self.vw.default_parts.add(self.sunroof)
    models.signals.m2m_changed.connect(self.m2m_changed_signal_receiver, Car.default_parts.through)
