Show Gtk::TreeModelColumn<long> as appropriate strings and choose them by Gtk::CellRendererCombo.

[O01eg <o01eg yandex ru>]

I write GUI to access to database. Some tables have foreign keys.
I mapped Gtk::TreeModel and Gtk::TreeModelColumnRecord to SQL and can
edit simple values but cann't write Gtk::CellRenderer to show
Gtk::TreeModelColumn<long> of foreign key as
Gtk::TreeModelColumn<Glib::ustring> of appropriate string from foreign
table.
Is it possible or I need add string column and work with strings when
accessing to database?

My current code: http://ompldr.org/vODAxdg