@PostPersist @OneToMany mappedBy问题

Question

这是我的课程，

@Entity
class Advertisement {
    @Id
    @GeneratedValue
    private long id;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "advertisement")
    private List<AdvertisementCoupon> coupons = new ArrayList<>();

    @PostPersist
    public void onPostPersist() {
 EventPublisher.CURRENT.publishEvent(new AdvertisementCreatedEvent(this));
    }
}

@Entity
class AdvertisementCoupon {
    @Id
    @GeneratedValue
    private long id;

    @ManyToOne
    private Advertisement advertisement;
}

class TimelineEventListener {

    @EventListener
    public void onAdvertisementCreated(AdvertisementCreatedEvent event){

        if (timelineRepository.existsByUserIdAndAdvertisementId(
            event.getAdvertisement().getCreatedUser().getId(), 
            event.getAdvertisement().getId())) {

            return;
        }

        TimelineAdvertisement timelineAdvertisement = new TimelineAdvertisement(event.getAdvertisement().getCreatedUser(), event.getAdvertisement());
        timelineRepository.save(timelineAdvertisement);
    }
}

调用@PostPersist时，只插入广告（我打印sql）。

在onAdvertisementCreated方法中，当调用timelineRepository.existsByUserIdAndAdvertisementId时，会插入AdvertisementCoupon，但会引发Column 'advertisement_id' cannot be null异常。 广告ID已生成。

如何绕过这个问题，谢谢。