Entity Framework 4.0: On the way to Composite Pattern

In my last posting about Entity Framework 4.0, Entity Framework 4.0: POCOs and table-per-type inheritance mapping, I made first generalization to my photo gallery model. I introduced GalleryItem class that is base class for all that can be added to gallery: new albums, photos and maybe videos too. Although everything works as expected my model needs some modifications because it is not very foolproof.