QDataWidgetMapper is a class that allows us to map the widgets in a form to the corresponding columns in a data model.

tableModel = new QSqlRelationalTableModel(this);
tableModel->setTable("employee");
tableModel->setRelation(Employee_DepartmentId, QSqlRelation("department", "id", "name"));
tableModel->setSort(Employee_Name, Qt::AscendingOrder);
tableModel->select();

QSqlTableModel *relationModel = tableModel->relationModel(Employee_DepartmentId);
departmentComboBox->setModel(relationModel);
departmentComboBox->setModelColumn(relationModel->fieldIndex("name"));

mapper = new QDataWidgetMapper(this);
mapper->setSubmitPolicy(QDataWidgetMapper::AutoSubmit);
mapper->setModel(tableModel);
mapper->setItemDelegate(new QSqlRelationalDelegate(this));
mapper->addMapping(nameEdit, Employee_Name);
mapper->addMapping(departmentComboBox, Employee_DepartmentId);
mapper->addMapping(extensionLineEdit, Employee_Extension);
mapper->addMapping(emailEdit, Employee_Email);
mapper->addMapping(startDateEdit, Employee_StartDate);