Hi, I am bit puzzled with this. I am looking at a codebase, where there are multiple joined-subclass mappings (in separate hbm.xml files) which map to same table. For union-subclass, we correctly get exception for duplicate mapping. But for joined-subclass it allows multiple mappings to same table. In the hibernate source code, I see that for Joined subclass, the table mapping is added with following method call. Table mytable = mappings.addTable( schema, catalog, getClassTableName( joinedSubclass, node, schema, catalog, null, mappings ), getSubselect( node ), false For union-subclass, the mapping gets added with Table mytable = mappings.addDenormalizedTable( schema, catalog, getClassTableName(unionSubclass, node, schema, catalog, denormalizedSuperTable, mappings ), unionSubclass.isAbstract() != null && unionSubclass.isAbstract(), getSubselect( node ), denormalizedSuperTable ); mappings.addTable doesnt throw exception if the table mapping already exists. Is the behaviour for joined subclass by design? Because I see this fact being used in some projects, where to extend an already created joined-subclass, a new hibernate-config file is added with joined-subclass definition duplicated, with additional attributes (and columns) added to existing table. So essentially, you have two joined-subclass definitions, mapping two separate entities to same table. This does look like using some loop hole in the code? Thanks, Unmesh