How to write a prepared update query? - c++

Question

I've been trying to write an update function that takes the user to be updated(usr), the flag which then translates to the field to be updated and the new value. Here's the function,
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;
}
When I run the code I get an error. Here is the output:
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
How to fix this?

Answer 1

Cause
First problem
It is not hard to guess, that value binding works only for values, not for field names.
For more info, please read about Inserting, Updating, and Deleting Records.
Second problem
Found by #JimCastro:
You have a typo in your query. UPADATE should be UPDATE.
Solution
Use a string concatenation to form your prepared query.
Example
Instead of
query.prepare("UPDATE Users SET :field = :newValue WHERE Username = :Username");
write
query.prepare("UPDATE Users SET " + field + " = :newValue WHERE Username = :Username");
Delete or comment this line
query.bindValue(":field",field);