I am mapping a legacy database model with Hibernate. Now I got an
ConstraintViolationException. I think it is a bug. Here is simple test case which show my problem:

Hibernate version:
3.0.4/3.05

Mapping documents:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="test">

<class name="Parent" lazy="true" table="T_PARENT">
<composite-id name="key">
<key-property name="id1"/>
<key-property name="id2"/>
</composite-id>
<property name="name"/>
<set name="childs" lazy="true" order-by="id3" cascade="all">
<key>
<column name="id1"/>
<column name="id2"/>
</key>
<one-to-many class="Child"/>
</set>
</class>

<class name="Child" lazy="true" table="T_CHILD">
<composite-id name="key">
<key-property name="id1" />
<key-property name="id2" />
<key-property name="id3" />
</composite-id>
<property name="name" />
</class>

</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():
Parent parent = new Parent();
parent.setKey(new Key(1, 1));
parent.setName("Parent");
Child child = new Child();
child.setKey(new Key(1, 1, 0));
child.setName("Child");
parent.getChilds().add(child);

session.save(parent);
session.flush();

parent.getChilds().remove(child);
session.flush();

Full stack trace of any exception that occurs:
org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:63)
at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:179)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:226)
at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:139)
at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:274)
at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:27)
at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:726)
at test.BugTest.testXX(BugTest.java:57)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:474)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:342)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:194)
Caused by: Batch-Anweisung Nummer 0 (update T_CHILD set id1=null, id2=null where id1=) wurde abgebrochen.
at org.postgresql.jdbc2.AbstractJdbc2Statement.executeBatch(AbstractJdbc2Statement.java:107)
at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:57)
at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:172)
... 21 more

Name and version of the database you are using:
Postgresql 7.4, Oracle 9

The generated SQL (show_sql=true):
Hibernate: select child_.id1, child_.id2, child_.id3, child_.name as name1_ from T_CHILD child_ where child_.id1=? and child_.id2=? and child_.id3=?
Hibernate: insert into T_PARENT (name, id1, id2) values (?, ?, ?)
Hibernate: insert into T_CHILD (name, id1, id2, id3) values (?, ?, ?, ?)
Hibernate: update T_CHILD set id1=?, id2=? where id1=? and id2=? and id3=?
Hibernate: update T_CHILD set id1=null, id2=null where id1=? and id2=?