QDataWidgetMapper with a QComboBox and a QSortFilterProxyModel doesn't work

I’m using QDataWidgetMapper to map a column from a QSortFilterProxyModel to a QComboBox.

QSqlRelationalTableModel *m_Model;
QSortFilterProxyModel *m_ProxyModel;
m_ProxyModel->setSourceModel(m_Model);

mapper = new QDataWidgetMapper(this);
mapper->setModel(m_ProxyModel);

QSqlTableModel *relModel = m_Model->relationModel(typeIndex);
projektComboBox->setModel(relModel);
projektComboBox->setModelColumn(relModel->fieldIndex("projectID");
mapper->addMapping(projektComboBox, typeIndex);

The values is displayed correctly in the ComboBox. But changes are not written to m_ProxyModel.

If I use the QSqlRelationalTableModel for mapping instead QSortFilterProxyModel for the mapping, everything works fine.

Another column mapped on a QLineEdit works fine with both: QSqlRelationalTableModel and QSortFilterProxyModel). Only QComboBox has the problem.

I upload a sample: "modelproblem.zip".

In the sample you can change the used model (QSqlRelationalTableModel/QSortFilterProxyModel) in line 23/24 in mainwindow.cpp