I have got a class

class Product extends DataObject {
  static $many_many = array('categories'=>'Category');
}

and

class Category extends DataObject {
  static $belongs_many_many = array('categories'=>'Category' );
  static $has_one = array('Parent' =>'Category');
}

I would like to filter products by sub-categories only, If I put

 $searchable_fields = array('Categories.ID' ) 

in the Product class, i will have a dropdown box list all categories (including the Parent ones), which is messy