hibernate4 外键级联删除cascade="delete"属性失效不起作用

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hibernate4 外键级联删除cascade="delete"属性失效不起作用5

下面是我的两个映射文件：
Blog：
<class name="Blog" table="blog">
<id name="id" column="id">
<generator class="identity"></generator>
</id>
<property name="title" type="string" column="title"></property>
<property name="content" type="string" column="content"></property>
<property name="datetime" type="date" column="createTime"></property>
<set name="comments" inverse="true" cascade="delete">
<key column="blogId"></key>
<one-to-many class="Comment"/>
</set>
</class>

Comment：
<class name="Comment" table="comment">
<id name="id" column="id">
<generator class="identity"></generator>
</id>
<property name="user" type="string" column="user"></property>
<property name="email" type="string" column="email"></property>
<property name="url" type="string" column="url"></property>
<property name="content" type="string" column="content"></property>
<property name="datetime" type="date" column="createTime"></property>
<many-to-one name="blog" class="Blog" column="blogId" not-null="true"></many-to-one>
</class>

当我运行程序的时候打印出来hibernate执行的sql语句是这样的：
Hibernate:
    alter table comment
        drop
        foreign key FK38A5EE5FDE0F1321
Hibernate:
    drop table if exists blog
Hibernate:
    drop table if exists comment
Hibernate:
    create table blog (
        id integer not null auto_increment,
        title varchar(255),
        content varchar(255),
        createTime date,
        primary key (id)
    ) type=InnoDB
Hibernate:
    create table comment (
        id integer not null auto_increment,
        user varchar(255),
        email varchar(255),
        url varchar(255),
        content varchar(255),
        createTime date,
        blogId integer not null,
        primary key (id)
    ) type=InnoDB
Hibernate:
    alter table comment
        add index FK38A5EE5FDE0F1321 (blogId),
        add constraint FK38A5EE5FDE0F1321
        foreign key (blogId)
        references blog (id)

而我认为正确的情况下。最后一节添加外键关系的SQL语句应该是这样的：
    alter table comment
        add index FK38A5EE5FDE0F1321 (blogId),
        add constraint FK38A5EE5FDE0F1321
        foreign key (blogId)
        references blog (id)
        on delete cascade

结果我在删除Blog的时候抛出如下异常：
Cannot delete or update a parent row: a foreign key constraint fails (`blog/comment`, CONSTRAINT `FK38A5EE5FDE0F1321` FOREIGN KEY (`blogId`) REFERENCES `blog` (`id`))

我程序的main方法具体如下：
public static void main(String[] args) {
Session session = HibernateUtil.currentSession();
Blog blog = new Blog();
blog.setTitle("a title");
session.save(blog);
Comment comment = new Comment();
comment.setContent("good");
comment.setBlog(blog);
session.save(comment);
session.delete(blog);
session.flush();
HibernateUtil.closeSession();
}