Finally i found it. This mapping causes the exception: <?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 package="demo"> | <class name="Sample2Well" table="sample2well" optimistic-lock="none"> | <id name="id" type="long" unsaved-value="null" column="SAMPLE2WELLID"> | <generator class="native"/> | </id> | <property name="analysisIndex" type="integer" column="analysisindex"/> | <bag name="sampleWell" lazy="false" table="samplewell" cascade="save-update"> | <key foreign-key="sampleWell_id" column="SAMPLE2WELLID"/> | <composite-element class="demo.SampleWellType"> | <many-to-one name="sample" entity-name="demo.Sample" foreign-key="Sample_id" not-null="false" column="SAMPLEID" cascade="none"/> | <nested-composite-element name="well" class="demo.Well"> | <property name="adress" type="string" column="well_adress"/> | <property name="column" type="short" column="well_column"/> | <property name="row" type="character" column="well_row" length="1"/> | </nested-composite-element> | </composite-element> | </bag> | </class> | </hibernate-mapping> | <hibernate-mapping package="demo"> | <class name="Sample" table="sample" optimistic-lock="none"> | <meta attribute="class-description" inherit=&quot;false&quot;&gt;(a)hibernate.class table="sample"</meta> | <id name="id" type="long" unsaved-value="null" column="SAMPLEID"> | <generator class="native"/> | </id> | <version name="version" column="version"/> | <property name="sampleCode" type="string" column="sample_code"/> | </class> | </hibernate-mapping> | I have created this second jira issue: http://jira.jboss.com/jira/browse/JBIDE-1768 View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4128158#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...