Why would be a QIdentityProxyModel easier?

I still will have problems with an index.
because (set)data works with them. Which index will have the extra column? It still needs to be maped by maped to and from source.

I also think that ToSource and FromSource need to be reimplemented, because QIdentityProxyModel works with 1 source model. My subclass needs to work with 1 source model and an extra column.