I did it once like this for a static QTreeWidget:
Qt Code: 
  1. void saveTreeState()
  2. {
  3.     QVariantList states;
  4.     QTreeWidgetItemIterator it(treeWidget);
  5.     while (*it)
  6.     {
  7.         states += (*it)->isExpanded();
  8.         ++it;
  9.     }
  10.     QSettings settings;
  11.     settings.setValue("states", states);
  12. }
  13.  
  14. void restoreTreeState()
  15. {
  16.     QSettings settings;
  17.     QVariantList states = settings.value("states").toList();
  18.     QTreeWidgetItemIterator it(treeWidget);
  19.     while (*it && !states.isEmpty())
  20.     {
  21.         (*it)->setExpanded(states.takeFirst().toBool());
  22.         ++it;
  23.     }
  24. }
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Of course, it doesn't apply in your situation Patrik because you're using a model, not QTreeWidget. But perhaps it turns out to be useful for someone who later searches for a solution on the same subject.