Hibernate is updating instead of inserting, why?

I'm starting my adventure with Hibernate, so please be patient :) I want to make mapping for two tables, for example A and B. The relation beetwen A and B is one-to-many. I wrote this hbm.xml file:

 <hibernate-mapping package="something">
   <class name="A" table="A">
   <id name="id" type="int" column="ID">
      <generator class="native" />
   </id>
   <set name="setInA" sort="natural" cascade="all" lazy="false">
     <key column="ANOTHER_ID"/>
     &l开发者_如何学运维t;one-to-many class="B" />
   </set>
   </class>

  <class name="B" table="B">
   <id name="anotherId" type="int" column="ANOTHER_ID">
      <generator class="native" />
   </id>
   </class>
</hibernate-mapping>

Of course I made also POJO classes A and B.

And now, when I try to do:

A a = new A();
Set<B> set = new TreeSet<B>();
set.add(new B());
a.setSetInA(set);
session.save(a);

Hibernate inserts new row to table A, but (what is the worst) is not inserting new row to B table, but only makes SQL Update on not existing row in B. Can tell me anyone why it is happening? What I made wrong?

You should either persist B's objects firstly, or use Cascade option.

You can use Cascade without using annotations:

<set name="setInA" sort="natural" cascade="all" lazy="false" cascade="all">
     <key column="ANOTHER_ID"/>
     <one-to-many class="B" />
   </set>

This will ensure that collection of B instances is inserted when you insert A.

Found this question while searching for causes of the same symptoms in my system. cascade="all" did not help.

In my case I solved this by adding a mapping to the list element, in this example class B.

Please note that the enclosing class (A in this example) also was versioned. Hibernate might require that versioning (used for optimistic locking) must be enabled for all nested elements. I haven't found any documentation to support this theory.