我一直在尝试编写一个更新函数，该函数包含要更新的用户(usr)，然后转换为要更新的字段的标志和新值。 这是函数，

bool DBManager::updateDB(User* usr, int flag, QVariant& value)
{
    QSqlQuery query;
    bool status = false;
    QString temp_value = "";
    QList<QString> fieldlist = {"First_Name", "Last_Name", "Email", "Username", "Password"};
    QString field = fieldlist.at(flag);

    if (field == "First_Name") {
        usr->setFirstName(value.toString());
        temp_value = usr->getFirstName();
    } else if (field == "Last_Name") {
        usr->setLastName(value.toString());
        temp_value = usr->getLastName();
    } else {
        qDebug() << "invalid option";
    }

    query.prepare("UPADATE Users SET :field = :newValue WHERE Username = :Username");
    query.bindValue(":field",field);
    query.bindValue(":newValue",temp_value);
    query.bindValue(":Username",usr->getUserName());
    qDebug() << query.boundValues();
    if (m_db.isOpen()){
        if (query.exec()){
            status = true;
            qDebug() << "DATABASE:: Record update successful";
        } else {
            qDebug() << "DATABASE:: Record update unsuccessful - " << query.lastError();
        }
    } else {
        qDebug() << "DATABASE:: not open ";
    }

    return status;
}

当我运行代码时，我得到一个错误。 以下是输出:

QMap((":Username", QVariant(QString, "zielinkin1"))(":field", QVariant(QString, "Last_Name"))(":newValue", QVariant(QString, "Thibos")))

DATABASE:: Record update unsuccessful -  QSqlError("", "Parameter count mismatch", "") User not updated DATABASE:: database closed

怎么解决这个问题？