Optional belongsTo relationship in cakephp

Hi SO,

I have a simple Category model in my CakePHP application. I want to add sub-categories, and do this by simply adding a parent_id column.

The parent_id is a belongsTo relationship, that references back to the same Category model.

When I generate my admin scaffolds, a dropdown will shop up (correct!), but I'd like to include a NULL option in this dropdown (for top-level categories).

Is this possible? And if so, how..

(Note: not interested in Tree behaviour right now)