after two days of debugging, it seems the embedded hsql database in JBoss4.2.2 creates a wrong result set from the join statement. The statement: | select | person0_.ID as ID0_0_, | person0_.NAME as NAME0_0_, | person0_.SSN as SSN1_0_, | person0_.clazz_ as clazz_0_ | from | ( | select | null as SSN, | NAME, | ID, | 0 as clazz_ | from PERSON union select | SSN, | NAME, | ID, | 1 as clazz_ | from Employee | ) | person0_ where | person0_.ID=? results in a unexpected NULL value in the SSN Field of the resultset. which causes Hibernate to produce the PropertyAccessError. with the hsqldb embedded in jboss 4.0.5 the statement produces a correct resultset. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4131675#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...