hibernate探索-级联删除

Hibernate在级联删除前要先把外键字段置为null，如果这个外键字段不允许为空，则会报错。 

需要在many端设置参数：inverse=“true”，hibernate文档中有如下描述：

Very Important Note: If the <key> column of a <one-to-many> association is declared NOT NULL, Hibernate may cause constraint violations when it creates or updates the association. To prevent this problem, you must use a bidirectional association with the many valued end (the set or bag) marked as inverse="true".