Partially update GroupsModelArray for Grid or ListBox

There is no documented way to update a GroupsModelArray partially on a per row basis.

http://zkfiddle.org/sample/2ab6m44/2-Partially-update-GroupModel

For example what woulf I do to update a single row with the partialbtn in the example fiddle.

So far the only way I see is to roll a custom group model - is this right?