data, model and tree view

[wysota]

You mean instead of using (e.g. in data)
return sourceModel()->data(index, role);

I should better use
return sourceModel()->data(mapToSource(index), role);

Yes. And the same in all other places. Remember that indexes are only valid in context of a current model. You can't "share" or "move" indexes between models. And you can't store them too (you have QPersistentModelIndex for that).