Need help with Hibernate? Read this first:
http://www.hibernate.org/ForumMailingli ... AskForHelp

Hibernate version:
3.0
Mapping documents:
<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping
>
<class
name="com.ingenix.freya.rulesmanagement.model.RuleLiteImpl"
table="RULEARTIFACTDTLS"
lazy="false"
>

<id
name="ID"
column="UUID"
type="java.lang.String"
unsaved-value="null"
>
<generator class="native">
<!--
To add non XDoclet generator parameters, create a file named
hibernate-generator-params-RuleLiteImpl.xml
containing the additional parameters and place it in your merge dir.
-->
</generator>
</id>

<property
name="ruleName"
type="java.lang.String"
update="true"
insert="true"
column="name"
/>

<property
name="lastChangedOn"
type="java.util.Date"
update="true"
insert="true"
column="LastChangedOn"
/>

<property
name="effectiveDate"
type="java.util.Date"
update="true"
insert="true"
column="effectiveDate"
/>

<property
name="expirationDate"
type="java.util.Date"
update="true"
insert="true"
column="expirationDate"
/>

<property
name="description"
type="java.lang.String"
update="true"
insert="true"
column="documentation"
/>

<property
name="rulesetNames"
type="java.util.List"
formula="(select fr.name from freya_ruleset fr, ruleset_version rv, freya_ruleset_rule frr, ruleartifactdtls r where frr.rule_iid = r.uuid and rv.ruleset_version_iid = frr.ruleset_version_iid and fr.ruleset_iid = rv.ruleset_iid and r.uuid = UUID)"
update="false"
insert="false"
/>

<!--
To add non XDoclet property mappings, create a file named
hibernate-properties-RuleLiteImpl.xml
containing the additional properties and place it in your merge dir.
-->

</class>

</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():
tx = session.beginTransaction();
mLog.debug("Attempting to save: " + object);
result = session.save(object);
tx.commit();
Full stack trace of any exception that occurs:
org.hibernate.MappingException: Could not determine type for: java.util.List, for columns: [org.hibernate.mapping.Formula( (select fr.name from freya_ruleset fr, ruleset_version rv, freya_ruleset_rule frr, ruleartifactdtls r where frr.rule_iid = r.uuid and rv.ruleset_version_iid = frr.ruleset_version_iid and fr.ruleset_iid = rv.ruleset_iid and r.uuid = UUID) )]
at org.hibernate.mapping.SimpleValue.getType(SimpleValue.java:265)
at org.hibernate.mapping.SimpleValue.isValid(SimpleValue.java:252)
at org.hibernate.mapping.Property.isValid(Property.java:174)
at org.hibernate.mapping.PersistentClass.validate(PersistentClass.java:325)
at org.hibernate.mapping.RootClass.validate(RootClass.java:188)
at org.hibernate.cfg.Configuration.validate(Configuration.java:839)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1000)
at com.ingenix.freya.util.hibernate.HibernateUtil.<clinit>(HibernateUtil.java:50)
at com.ingenix.freya.enterprise.ejb.EnterpriseServiceImpl.getSession(EnterpriseServiceImpl.java:481)
at com.ingenix.freya.util.ejb.BasePersistenceService.executeHibernateQuery(BasePersistenceService.java:644)
at com.ingenix.freya.enterprise.ejb.EnterpriseServiceImpl.getAllEnterprises(EnterpriseServiceImpl.java:436)
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:585)
at org.jboss.invocation.Invocation.performCall(Invocation.java:345)
at org.jboss.ejb.StatelessSessionContainer$ContainerInterceptor.invoke(StatelessSessionContainer.java:214)
at org.jboss.resource.connectionmanager.CachedConnectionInterceptor.invoke(CachedConnectionInterceptor.java:185)
at org.jboss.ejb.plugins.StatelessSessionInstanceInterceptor.invoke(StatelessSessionInstanceInterceptor.java:130)
at org.jboss.webservice.server.ServiceEndpointInterceptor.invoke(ServiceEndpointInterceptor.java:51)
at org.jboss.ejb.plugins.CallValidationInterceptor.invoke(CallValidationInterceptor.java:48)
at org.jboss.ejb.plugins.AbstractTxInterceptor.invokeNext(AbstractTxInterceptor.java:105)
at org.jboss.ejb.plugins.TxInterceptorCMT.runWithTransactions(TxInterceptorCMT.java:335)
at org.jboss.ejb.plugins.TxInterceptorCMT.invoke(TxInterceptorCMT.java:166)
at org.jboss.ejb.plugins.SecurityInterceptor.invoke(SecurityInterceptor.java:139)
at org.jboss.ejb.plugins.LogInterceptor.invoke(LogInterceptor.java:192)
at org.jboss.ejb.plugins.ProxyFactoryFinderInterceptor.invoke(ProxyFactoryFinderInterceptor.java:122)
at org.jboss.ejb.SessionContainer.internalInvoke(SessionContainer.java:624)
at org.jboss.ejb.Container.invoke(Container.java:873)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:141)
at org.jboss.mx.server.Invocation.dispatch(Invocation.java:80)
at org.jboss.mx.server.Invocation.invoke(Invocation.java:72)
at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:249)
at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:644)
at org.jboss.invocation.jrmp.server.JRMPInvoker$MBeanServerAction.invoke(JRMPInvoker.java:805)
at org.jboss.invocation.jrmp.server.JRMPInvoker.invoke(JRMPInvoker.java:406)
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:585)
at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:294)
at sun.rmi.transport.Transport$1.run(Transport.java:153)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.Transport.serviceCall(Transport.java:149)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:460)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:701)
at java.lang.Thread.run(Thread.java:595)

Name and version of the database you are using:
SQL Server 2005
The generated SQL (show_sql=true):
No SQL generated yet. Seems to fails while processing associations...
Debug level Hibernate log excerpt:
None.

Obviously that formula is returning more than one row (hence a List). You can't do that. Put in a "top 1", or further restrict the where clause so that the subselect results in only one value.