GtkTreeView children missing when filterable and sortable

Hello,

I have a GtkTreeView where I wanted both custom filtering and the 
ability to sort by column.  Originally my model was a GtkListStore, but 
to include filtering I wrapped it in a GtkTreeModelFilter.  However, 
since GtkTreeModelFilter does not implement the GtkTreeSortable 
interface this made the columns unsortable by hiding the GtkListStore 
which does implement GtkTreeSortable.  So, to restore sortablity I 
further wrapped the GtkTreeModelFilter in a GtkTreeModelSort.  This 
worked correctly and I was able to make the GtkTreeView sortable and 
filterable at the same time.

The new problem I'm having results from changing to a GtkTreeStore as 
the base model.  Now I need parent-child relationships, but with this 
configuration (GtkTreeStore/GtkTreeModelFilter/GtkTreeModelSort) no 
child nodes ever appear and no expanders are shown for root nodes that 
have children. 

This seems like a bug.  All of the following configurations (several of 
which serve no practical purpose other than to illustrate they don't 
cause the problem) show the child nodes as expected:
GtkTreeStore
GtkTreeStore/GtkTreeModelFilter
GtkTreeStore/GtkTreeModelSort
GtkTreeStore/GtkTreeModelSort/GtkTreeModelFilter

Is there an expected (non-bug) reason this happens?
Is there a workaround?

-Jake Cobb

P.S. I've posted before about my attempt to combine the filtering and 
sorting into a single class from within PHP, but its been some time 
since that post so I'll repeat it here.

I implemented a custom class in PHP that extends GtkTreeModelFilter and 
implements GtkTreeSortable.  For each of the GtkTreeSortable methods, my 
class called the method of the underlying model.  This does not work 
because Gtk+, on the C side of things, will not recognize that you've 
implemented the sorting methods and complains with the following:

(php.exe:304): Gtk-CRITICAL **: gtk_tree_sortable_get_sort_column_id: 
assertion
`GTK_IS_TREE_SORTABLE (sortable)' failed