异常：org.hibernate.id.IdentifierGenerationException

在有关联关系的数据表中保存数据时，先保存一端，再保存多端的抛出的异常(此时不管一端，还是多端的对象都没有设置id,属性，也就是要保存的两个对象的id 属性为空。)

org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): com.zcd.hibernate.oneToOne.Department
at org.hibernate.id.Assigned.generate(Assigned.java:52)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId