QSqlQuery error handling

jsulm

@SPlatten said in QSqlQuery error handling:

encapsulates the text

Which text? We are talking about table and column names.

What does https://doc.qt.io/qt-5.15/qsqlquery.html#size return after query execution?

SPlatten

@jsulm , Is something wrong with this code:

    QSqlRecord record(query.record());
    int intTotal(record.count());
    if ( intTotal == 0 )
    {
        return;
    }
    if ( query.next() )
    {
        QSqlField field(record.field(0));
        QVariant varValue(field.value());
        if ( varValue.isValid() != true )
        {
            return;
        }
//Send binary data
        sendBinary(lngBlockNo, varValue.toByteArray());

SPlatten

@jsulm size returns 1. Which is correct, but field shows as "" in the debugger, varValue also shows as "" in debugger. When I get into the function sendBinary there is a check for the byte array:

if ( crbaData.length() == 0 ) {
    return;
}

It does return because the array length is 0.

jsulm

@SPlatten said in QSqlQuery error handling:

QSqlRecord record(query.record());
int intTotal(record.count());
if ( intTotal == 0 )
{
return;
}

Why do you need this?

while (query.next()) {
...
}

is enough.

SPlatten

@jsulm , ok, I can remove that, but that doesn't explain the problem.

jsulm

@SPlatten Also please read what https://doc.qt.io/qt-5.15/qsqlrecord.html#count actually returns

SPlatten

@jsulm , I've removed that check but still doesn't explain why there is no error and no data, yet the data exists and the query is correct.

jsulm

@SPlatten Do you get valid data with this:

while (query.next()) {
    QByteArray data = query.value(0).toByteArray();
    ...
}

?

SPlatten

@jsulm , thank you, I really don't know why but prior to today the code I posted was working, I've just modified it using your example and its ok.

KroMignon

@SPlatten AFAIK, QSqlQuery::record() is to be used after QSqlQuery::exec() to get the field information of the current query.
So you can find the indexes of the value and decode faster the results:

if(query.exec("SELECT * from MyTable where status = 2"))
{
     auto record = query.record();
     int nameCol = record.indexOf("name");
     while(query.next())
         qDebug() << "Found:" << query.value(nameCol ).toString();
}

Which is faster as using query.value("name").toString()