动态数据站点下拉列表中的分层数据

I'm getting into dynamic data sites (remarkably powerful), and enjoying it. However, I have a requirement that I can't seem to meet. I have a self-joined category table (Id, Name, ParentId) to create a hierarchical category structure (e.g. the categories with null ids are top level, usual sort of thing). I then have a products table that has a foreign key to the category.I'm getting into dynamic data sites (remarkably