attempted to assign id from null one-to-one property: person

Exception in thread "main" org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property: person
    at org.hibernate.id.ForeignGenerator.generate(ForeignGenerator.java:44)
    at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:99)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:94)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
    at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:507)
    at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:499)
    at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:218)
    at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:268)
    at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:216)
    at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:169)
    at org.hibernate.engine.Cascade.cascade(Cascade.java:130)
    at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:456)
    at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:334)
    at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:181)
    at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:107)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:187)
    at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:33)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:172)
    at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:27)
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
    at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:535)
    at org.hibernate.impl.SessionImpl.save(SessionImpl.java:523)
    at org.hibernate.impl.SessionImpl.save(SessionImpl.java:519)
    at cn.itcast.test.Demo3.add(Demo3.java:29)
    at cn.itcast.test.Demo3.main(Demo3.java:14)

我出现这个问题的原因是：

我在父表上加上了级联属性，然后我希望在保存person的时候自动保存idcard对象，忘记了附表是依赖于目标存在的，所以报错了。将级联添加到附表上了，就ok了