QSqlRelationalDelegate make only the job for sjow edited data... after that, it give back the id of foreign key. But why ?

uservalidation.cpp

void usersValidation::populateUsersToValid() {
    modeUsersToValid = new QSqlRelationalTableModel(this, modelDB->currentDB());
	formApprobation = new MyDelegate(this);
	formPrivileges = new QSqlRelationalDelegate(this);
    modeUsersToValid->setTable("\"UserNames\"");
    modeUsersToValid->setSort(USERNAMES_USERNAME, Qt::AscendingOrder);
    modeUsersToValid->setEditStrategy(QSqlRelationalTableModel::OnFieldChange);
    modeUsersToValid->setRelation(USERNAMES_ID_PRIVILEGE,
                                  QSqlRelation("\"Privileges\"",
                                  "id", "privilege"));
    modeUsersToValid->setHeaderData(USERNAMES_USERNAME, Qt::Horizontal,
                                    QObject::tr("Utilisateur"));
    modeUsersToValid->setHeaderData(USERNAMES_NAME, Qt::Horizontal,
                                    QObject::tr("Prénom"));
    modeUsersToValid->setHeaderData(USERNAMES_FAMILLYNAME, Qt::Horizontal,
                                    QObject::tr("Nom"));
    modeUsersToValid->setHeaderData(USERNAMES_ID_PRIVILEGE, Qt::Horizontal,
                                    QObject::tr("Privilège"));
    modeUsersToValid->setHeaderData(USERNAMES_APPROBATION, Qt::Horizontal,
                                    QObject::tr("Approuvé"));
    modeUsersToValid->setHeaderData(USERNAMES_DATECREATION, Qt::Horizontal,
                                    QObject::tr("Date"));
    modeUsersToValid->select();
    UsersToValid->setModel(modeUsersToValid);
	UsersToValid->setItemDelegateForColumn(USERNAMES_APPROBATION, formApprobation);
	UsersToValid->setItemDelegateForColumn(USERNAMES_ID_PRIVILEGE, formPrivileges);
    UsersToValid->setColumnHidden(USERNAMES_ID,			  true);
    UsersToValid->setColumnHidden(USERNAMES_PASSWORD,	  true);
    UsersToValid->setColumnHidden(USERNAMES_QUALIFICATIONS,true);
    UsersToValid
                 ->setSelectionBehavior(QAbstractItemView::SelectRows);
    UsersToValid->verticalHeader()->hide();
    UsersToValid->horizontalHeader()
                               ->setStretchLastSection(true);
    UsersToValid->resizeColumnsToContents();
}