表删除时 Cannot delete or update a parent row: a foreign key constraint fails 异常处理

有两张表，结构如下： 

t_item：                          t_bid：
id        int                     id        int
name    varchar                   name      varchar
                                  item_id   int

其中表t_item的主键id是表t_bid的item_id字段的外键。那么在这种情况下，如果删除表t_item中的记录，并且该记录中的id主键被t_bid中的item_id字段所引用，就会抛出如下异常： 

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`test/t_bid`, CONSTRAINT `fk_id` FOREIGN KEY (`id`) REFERENCES `t_item
` (`id`))

解决方法：级联删除，即在删除t_item表中的记录时同时删除t_bid表中的相关记录 。

(1) 增加外键约束时声明级联删除，即： 

alter table t_bid add constraint fk_id foreign key(id) references
key(id) on delete cascade；

(2) 使用触发器：在删除t_item表中记录之前先删除与之相关的t_bid表中的记录。 （这方法较好）
     触发器代码（MySQL）： 

delimiter //
create trigger tri_delete before delete on t_item
for each row
begin
     delete from t_bid where id = old.id;
end //

Hibernate中的解决方案： 
这个问题在Hibernate中相对容易解决，只需设置cascade = “delete”即可。此时观察发出的sql语句： 

Hibernate: select item0_.id as id0_0_, item0_.name as name0_0_ from t_item item0_ where item0_.id=?
Hibernate: select bids0_.item_id as item3_1_, bids0_.id as id1_, bids0_.id as id1_0_, bids0_.price as price1_0_, bids0_.item_id as item3_1_0_ from t_bid bids0_ where bids0_.item_id=?
Hibernate: update t_bid set item_id=null where item_id=?
Hibernate: delete from t_bid where id=?
Hibernate: delete from t_bid where id=?
Hibernate: delete from t_item where id=?

发现在删除t_bid表中记录之前会先将它的item_id字段值设置为null，但如果我们在映射文件中设置item_id字段不能为null，即设置Bid.hbm.xml文件为：

<many-to-one name="item" column="item_id" class="po.Item" not-null="true"/>

注意不能在Item.hbm.xml文件中进行如下设置(即在key元素中指定not-null="true")： 

<set name="bids" cascade="all">
    <key column="item_id" not-null="true"/>
    <one-to-many class="po.Bid"/>
</set>

这样会抛出"Repeated column in mapping for entity"异常
如果我们指定item_id字段值不能为null，那么在删除时会抛出如下异常：

org.hibernate.exception.GenericJDBCException: Could not execute JDBC batch update···
Caused by: java.sql.BatchUpdateException: Data truncation: Column set to default value; NULL supplied to NOT NULL column 'item_id' at row 1
•••

此时的解决方法是设置inverse="true"，这在Hibernate文档中有相应的描述：

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".

观察此时发出的sql语句：

Hibernate: select item0_.id as id1_0_, item0_.name as name1_0_ from t_item item0_ where item0_.id=?
Hibernate: select bids0_.item_id as item3_1_, bids0_.id as id1_, bids0_.id as id0_0_, bids0_.amount as amount0_0_, bids0_.item_id as item3_0_0_ from t_bid bids0_ where bids0_.item_id=?
Hibernate: delete from t_bid where id=?
Hibernate: delete from t_bid where id=?
Hibernate: delete from t_item where id=?

没有发出update语句。