two level RadTabStrip problem

[Alejandra]

Hi All, i´m trying to bind tabs from objects
The Second level tabs is loaded from a object collection that belongs to other object: 1st label-->Authors  and 2nd label-->Books (from an author)

I implemented a similar class to Declarative DataSources's example, and added the following code that gets the data (RadTabScript DataSocurce).

 List<SiteDataItem> siteData = new List<SiteDataItem>();
foreach (Author myauthor in GetAllAuthorsl()) {
   siteData.Add(new SiteDataItem(myauthor.Identity, null, myauthor.Name));
    foreach (Book myBook in GetAllBooks(myauthor.Identity))
        siteData.Add(new SiteDataItem myBook.Identity,myauthor.Identity, myBook.Name));
}

RadTabStrip properties are:
DataTextField = "Text" DataFieldID="ID" DataFieldParentID="ParentID"

The problem here is that all tabs are at the same level and the ID is not being set. 

any help would be grateful!