Problem is I am trying to do a One-to-Many uni directional mapping and get the following exception ' net.sf.hibernate.MappingException: No persister for: java.util.HashSet '

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Hibernate version using : Hibernate 2.1.6

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Mapping documents are as follows
Mapping Doc One:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="uk.gov.kn.ibio.bo.Briefcase" table="BRIEFCASE" schema="IBIO">
<id name="id" column="PK_ID" type="long">
<generator class="sequence">
<param name="sequence">IBIO.SEQ_BRIEFCASE</param>
</generator>
</id>
<property name="name" column="NAME" not-null="true" />
<property name="description" column="DESCRIPTION" not-null="true" />

<!-- uni-directional one-to-many association to Documents -->
<set name="documents" inverse="false" lazy="true" cascade="all" >
<key>
<column name="BR_DOC_FK_ID" />
</key>
<one-to-many class="uk.gov.kn.ibio.bo.Document" />
</set>
</class>
</hibernate-mapping>

Mapping Doc Two :

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="uk.gov.kn.ibio.bo.Document" table="DOCUMENT" schema="IBIO">
<id name="id" column="PK_ID" type="long">
<generator class="sequence">
<param name="sequence">IBIO.SEQ_DOC</param>
</generator>
</id>

<property name="documentReference" column="DOCUMENT_REF" not-null="true"/>
<property name="relvance" column="RELEVANCE" not-null="true"/>
<property name="summary" column="SUMMARY" not-null="true"/>
<property name="title" column="TITLE" not-null="true"/>
<property name="url" column="URL" not-null="true"/>

<many-to-one name="briefcase" column="BR_DOC_FK_ID" not-null="false"/>
</class>

</hibernate-mapping>
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Code between the session open and close the list of documents is alreday added to the briefcase before persisting.

session = sessionFactory.openSession();
transaction = session.beginTransaction();

session.save(briefcase);

transaction.commit();
} catch (HibernateException he) {
he.printStackTrace();
doRollback(transaction);

he.printStackTrace();
throw new UnresolvableStateException(
"Exception While Creating a IBIO SYSTEM " + he.getMessages() + " : " + this.getClass().getName());
} finally {
doClose(session);
}
}
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The Full stack trace is:
net.sf.hibernate.MappingException: No persister for: java.util.HashSet
at net.sf.hibernate.impl.SessionFactoryImpl.getPersister(SessionFactoryImpl.java:347)
at net.sf.hibernate.impl.SessionImpl.getClassPersister(SessionImpl.java:2690)
at net.sf.hibernate.impl.SessionImpl.getPersister(SessionImpl.java:2697)
at net.sf.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:1382)
at net.sf.hibernate.engine.Cascades$4.cascade(Cascades.java:114)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:436)
at net.sf.hibernate.engine.Cascades.cascadeCollection(Cascades.java:526)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:452)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:503)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:952)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:857)
at net.sf.hibernate.impl.SessionImpl.saveWithGeneratedIdentifier(SessionImpl.java:779)
at net.sf.hibernate.impl.SessionImpl.save(SessionImpl.java:738)
at uk.gov.kn.ibio.test.persistence.IBIODomainManagerTest.createBriefcaseForDocs(IBIODomainManagerTest.java:91)
at uk.gov.kn.ibio.persistence.tests.UserPersistenceTest.testCreateDocumentByBriefcase(UserPersistenceTest.java:92)
at java.lang.reflect.Method.invoke(Native Method)
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:392)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:276)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:167)
21:30:05,030 DEBUG SessionImpl:585 - transaction completion
21:30:05,030 DEBUG SessionImpl:573 - closing session
21:30:05,030 DEBUG SessionImpl:3336 - disconnecting session
net.sf.hibernate.MappingException: No persister for: java.util.HashSet
at net.sf.hibernate.impl.SessionFactoryImpl.getPersister(SessionFactoryImpl.java:347)
at net.sf.hibernate.impl.SessionImpl.getClassPersister(SessionImpl.java:2690)
at net.sf.hibernate.impl.SessionImpl.getPersister(SessionImpl.java:2697)
at net.sf.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:1382)
at net.sf.hibernate.engine.Cascades$4.cascade(Cascades.java:114)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:436)
at net.sf.hibernate.engine.Cascades.cascadeCollection(Cascades.java:526)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:452)
at net.sf.hibernate.engine.Cascades.cascade(Cascades.java:503)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:952)
at net.sf.hibernate.impl.SessionImpl.doSave(SessionImpl.java:857)
at net.sf.hibernate.impl.SessionImpl.saveWithGeneratedIdentifier(SessionImpl.java:779)
at net.sf.hibernate.impl.SessionImpl.save(SessionImpl.java:738)
at uk.gov.kn.ibio.test.persistence.IBIODomainManagerTest.createBriefcaseForDocs(IBIODomainManagerTest.java:91)
at uk.gov.kn.ibio.persistence.tests.UserPersistenceTest.testCreateDocumentByBriefcase(UserPersistenceTest.java:92)
at java.lang.reflect.Method.invoke(Native Method)
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:392)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:276)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:167)
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Name and version of the database you are using:
DB2 8.1
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The generated SQL (show_sql=true): SQL IS BELOW
21:30:04,999 DEBUG BatcherImpl:200 - about to open: 0 open PreparedStatements, 0 open ResultSets
21:30:04,999 DEBUG SQL:226 - values nextval for IBIO.SEQ_BRIEFCASE
Hibernate: values nextval for IBIO.SEQ_BRIEFCASE
21:30:04,999 DEBUG BatcherImpl:249 - preparing statement
21:30:05,015 DEBUG SequenceGenerator:81 - Sequence identifier generated: 1000025
21:30:05,015 DEBUG BatcherImpl:207 - done closing: 0 open PreparedStatements, 0 open ResultSets
21:30:05,015 DEBUG BatcherImpl:269 - closing statement
21:30:05,015 DEBUG SessionImpl:778 - generated identifier: 1000025
21:30:05,015 DEBUG SessionImpl:825 - saving [uk.gov.kn.ibio.bo.Briefcase#1000025]
21:30:05,015 DEBUG Cascades:497 - processing cascades for: uk.gov.kn.ibio.bo.Briefcase
21:30:05,015 DEBUG Cascades:506 - done processing cascades for: uk.gov.kn.ibio.bo.Briefcase
21:30:05,015 DEBUG WrapVisitor:81 - Wrapped collection in role: uk.gov.kn.ibio.bo.Briefcase.documents
21:30:05,015 DEBUG Cascades:497 - processing cascades for: uk.gov.kn.ibio.bo.Briefcase
21:30:05,015 DEBUG Cascades:524 - cascading to collection: uk.gov.kn.ibio.bo.Briefcase.documents
21:30:05,030 DEBUG Cascades:113 - cascading to saveOrUpdate()
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Debug level Hibernate log excerpt:
as above