How to update schema by adding Annotated classes to an existing Persistence unit in JPA with hibernate?

Environment: JPA 2.0, Hibernate 3.6, in a war file (with jsf 2.0, on jboss 6, etc).

I want update schema by adding Annotated classes to an existing Persistence unit in JPA with hibernate. this is what I have tried to do:

public void updateConfiguration() {
    // load classes
    Ejb3Configuration ejb3Configuration = new Ejb3Configuration();
    ejb3Configuration = ejb3Configuration.configure("existingPU", null);
    loadModel(ejb3Configuration);
    updateSchema(ejb3Configuration.getHibernateConfiguration());
}

private void updateSchema(Configuration cfg) {
    SchemaUpdate schemaUpdate = new SchemaUpdate(cfg);
    schemaUpdate.execute(true, true);
}

private void loadModel(Ejb3Configuration ejb3Configuration) {
    for (Object _object : objectList) {
        ejb3Configuration.addAnnotatedClass(_object.getClass());
    }

}

The existingPU is loaded by the normal JPA way using persistence.xml etc. The schema is updated for the entities in WEB-INF/classes and that part of the code base works perfectly. Now, the new classes in the objectList extend a @MappedSuperclass from WEB-INF/classes which has

@MappedSuperclass
public abstract class OwnerHolder implements Serializable {
    private User owner;
    @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "fk_owner_id")
    public User getOwner() {
         return owner;
    }

    public void setOwner(User owner) {
        this.owner= owner;
    }
}

This class is also extended by classes in WEB-INF/classes and that code works fine. The error I get is as follows:

13:36:45,494 ERROR [org.hibernate.tool.hbm2ddl.SchemaUpdate] could not complete schema update: org.hibernate.AnnotationException: @OneToOne or @ManyToOne on com.myproject.external.model.PurchaseOrder.owner references an unknown entity: com.myproject.model.User
at org.hibernate.cfg.ToOneFkSecondPass.doSecondPass(ToOneFkSecondPass.java:107) [:3.6.0.Final]
at org.hibernate.cfg.Configuration.processEndOfQueue(Configuration.java:1550) [:3.6.0.Final]
at org.hibernate.cfg.Configuration.processFkSecondPassInOrder(Configuration.java:1473) [:3.6.0.Final]
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1389) [:开发者_开发百科3.6.0.Final]
at org.hibernate.cfg.Configuration.generateSchemaUpdateScript(Configuration.java:1160) [:3.6.0.Final]
at org.hibernate.tool.hbm2ddl.SchemaUpdate.execute(SchemaUpdate.java:187) [:3.6.0.Final]
at com.myproject.MyConfiguration.updateSchema(MyConfiguration.java:43) [:]

The objectList is list of classes from a jar file placed in WEB-INF/lib.

Can you please help me?

Check if the User class hast the @Entity annotation in its declaration.