hibernate cascading delete, why not one delete on the foreign key?

I'm wondering why hibernate generates 1 delete per entity on a child table instead of using one delete on the foreign key

Here's the hibernate.cfg.xml (No i's not the next SO :-t

<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.url">jdbc:hsqldb:file:testdb;shutdown=true</property>
        <property name="hibernate.connection.driver_class">org.hsqldb.jdbcDriver</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.connection.pool_size">0</property>
        <property name="hibernate.dialect">org.hibernate.dialect.HSQLDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.format_sql">true</property>
        <property name="hbm2ddl.auto">auto</property>
        <mapping file="entities/Question.hbm.xml"/>
        <mapping file="entities/Answer.hbm.xml"/>

    </session-factory>

Question.hbm.xml

<hibernate-mapping>
   <class name="entities.Question">
      <id name="id">
         <generator class="native" />
      </id>
      <property name="title" not-null="true">
      </property>

      <property name="question" type="text" not-null="true">
      </property>

      <bag name="answers" inverse="true" cascade="all,delete-orphan" >
         <key>
            <column name="questionId" index="answer_questionId_idx" not-null="true"/>
         </key>
         <one-to-many class="entities.Answer"  />
      </bag>

      <property name="created" update="false" >
         <column name="created" not-null="true" index="answer_created_idx"></column>
      </property>
      <property name="lastUpdated">
         <column name="lastUpdated"  not-null="true" index="answer_lastUpdated_idx"></column>
      </property>
   </class>
</hibernate-mapping>

Answer.hbm.xml

<hibernate-mapping>
   <class name="entities.Answer">
      <id name="id">
         <generator class="native" />
      </id>

      <property name="answer" type="text" not-null="true">
      </property>

      <property name="created" update="false" >
            <column not-null="true" name="created" index="question_created_idx"></column>
      </property>

      <property name="lastUpdated" >
            开发者_如何转开发<column name="lastUpdated" not-null="true" index="question_lastUpdated_idx"></column>
                  </property>

      <many-to-one  name="question" column="questionId" not-null="true" update="false">
      </many-to-one>
   </class>
</hibernate-mapping>

There's 1 Question and 2 answers in my database, this test code:

Session session = factory.openSession();
Transaction t = session.beginTransaction();
Question q = (Question) session.load(Question.class,1);
session.delete(q);
t.commit();
session.close();

I would expect it to generate SQL like,

select .... from Questions where id = 1;
delete from Answers where questionId=1;
delete from Question where id=1;

I.e., just issue one delete to do the cascading delete on Answers, instead it's loading all the answers and issuing one delete per answer, like:

select
    question0_.id as id0_0_,
    question0_.title as title0_0_,
    question0_.question as question0_0_,
    question0_.created as created0_0_,
    question0_.lastUpdated as lastUpda5_0_0_ 
from
    Question question0_ 
where
    question0_.id=?

select
    answers0_.questionId as questionId0_1_,
    answers0_.id as id1_,
    answers0_.id as id1_0_,
    answers0_.answer as answer1_0_,
    answers0_.created as created1_0_,
    answers0_.lastUpdated as lastUpda4_1_0_,
    answers0_.questionId as questionId1_0_ 
from
    Answer answers0_ 
where
    answers0_.questionId=?

delete   from   Answer  where     id=?
delete   from   Answer  where     id=?
delete   from   Question where     id=?

How come, and is there anything I can do about it ?

Edit, in response to Nate Zaugg, I can get the db to do the cascading delete by setting on-delete="cascade" on the one-to-many key mapping, i'm more wondering why hibernate does what it does and not does one delete on the Answers table, and wheter threre's something wrong with my mappings.

Can you not configure your DMBS to do cascading deletes on relationships? It's really easy to do.

Edit: Try this <one-to-many class="entities.Answer" lazy="false" cascade="all" />