code: public List<PersistenceUnitRefType<WebAppDescriptor>> getAllPersistenceUnitRef()
     {
        List<PersistenceUnitRefType<WebAppDescriptor>> list = new ArrayList<PersistenceUnitRefType<WebAppDescriptor>>();
        List<Node> nodeList = model.get("persistence-unit-ref");
        for(Node node: nodeList)
        {
           PersistenceUnitRefType<WebAppDescriptor>  type = new PersistenceUnitRefTypeImpl<WebAppDescriptor>(this, "persistence-unit-ref", model, node);
           list.add(type);
        }
        return list;
     }
docs: Returns all <code>persistence-unit-ref</code> elements
  @return list of <code>persistence-unit-ref</code>