read a value from QSql database

[ODБOï]

I just created a database table, it looks like this

QSqlQuery query("CREATE TABLE tools (ind INTEGER, nom TEXT, longueur REAL, diametre REAL, dureeVieMinutes INTEGER, dureeUsageMinutes INTEGER)");

i wrote one methods for insert a tool and one edit a property from existing tool

i want to implement a method to read single property but i have troubles to do it

qreal ToolDBManager::getToolLength(QString _toolName){
    qreal length = -999; // return this if we cant read the length

    QSqlQuery query;
    query.prepare("SELECT longueur FROM tools WHERE nom=:toolName");
    query.bindValue(":toolName",_toolName);

    bool toRealOk = false; 

    if(query.exec()){

       length = query.record().value("longueur").toReal(&toRealOk); 
       qDebug()<< "LENGTH IS : " << length;

    }
    else
    {
         qDebug() << "getToolLength error:  "
                  << query.lastError();
    }
  
}

i alwase get 0.., witch is not the correct value set in the db
is there something not correct in this line

length = query.record().value("longueur").toReal(&toRealOk); 

can someone help please ?
thx

[JonB]

@LeLev
Let's start with:

• You're supposed to check if you got a row back. Just because the exec() succeeds does not mean it found a row. In which case, I suspect your query.record() would still be the "empty" record for the table.

• You're supposed to check what is in toRealOk after .toReal(&toRealOk), to tell you whether the conversion succeeded. I don't see you do that.

• To debug, start by just printing out what you get back from query.record().value("longueur"), without trying to toReal() it.

These apart, my guess is:
https://doc.qt.io/qt-5/qvariant.html#toReal

Returns the variant as a qreal if the variant has userType() QMetaType::Double, QMetaType::Float, QMetaType::Bool, QMetaType::QByteArray, QMetaType::Int, QMetaType::LongLong, QMetaType::QString, QMetaType::UInt, or QMetaType::ULongLong; otherwise returns 0.0.

At some level, the database table's REAL perhaps does not correspond to/has not been converted one of these Qt types (print out the QVariant type?). You have to look up how that bit works, database types != code types.

[ODБOï]

hi @JonB thank you very much

   if(query.exec()){
        qDebug()<< "RECORD VALUE IS : " <<  query.record().value("longueur");

this will output

RECORD VALUE IS :  QVariant(double, 0)

[mrjj]

Hi
It might not be placed on a valid row yet. (as @JonB also talk about)
The normal way as docs shows is

   QSqlQuery query("SELECT country FROM artist");
    while (query.next()) {
        QString country = query.value(0).toString();
        doSomething(country);
    }

where you call query.next() to place it on row.

[ODБOï]

@mrjj said in read a value from QSql database:

next()

yes exactly, thank you 2

qreal ToolDBManager::getToolLength(QString _toolName){
    qreal length = -999;

    QSqlQuery query;
    query.prepare("SELECT longueur FROM tools WHERE nom=:toolName");
    query.bindValue(":toolName",_toolName);

    bool toRealOk = false;

    if(query.exec()){

        while (query.next()) {

            length = query.record().value("longueur").toReal(&toRealOk);

            if(!toRealOk) return -999;
            
        }
    }
    else
    {
        qDebug() << "getToolLength error:  "
                 << query.lastError();
    }
    return length;
}

thank you very much @JonB @mrjj