Есть такой код
seriesIdx = dynamic_cast<MainWindow*>(parent)->getSeriesIdx();
QSqlTableModel *seriesModel = booksModel->relationModel(seriesIdx);
ui->seriesComboBox->setModel(seriesModel);
ui->seriesComboBox->setModelColumn(seriesModel->fieldIndex("series"));
mapper = new QDataWidgetMapper(this);
mapper->setSubmitPolicy(QDataWidgetMapper::ManualSubmit);
mapper->setModel(booksModel);
mapper->setItemDelegate(new QSqlRelationalDelegate(this));
mapper->addMapping(ui->seriesComboBox, seriesIdx);
booksModel->setData(booksModel->index(newRow, booksModel->fieldIndex("title")), ui->titleLineEdit->text());