QSqlQuery not working

[BrainStorm]

If you modify your codes to the following:

int ProductsForm::categoryIdByName(const QString& name)
{
    QSqlQuery categories(model->database());
    categories.exec(QString("SELECT id FROM Categories WHERE name='%1'").arg(name));
    if(categories.size()==-1)
    {
    qDebug() << "error";
    return 1;
    }
    .....
}

int ProductsForm::categoryIdByName(const QString& name)
{
    QSqlQuery categories(model->database());
    categories.exec(QString("SELECT id FROM Categories WHERE name='%1'").arg(name));
    if(categories.size()==-1)
    {
    qDebug() << "error";
    return 1;
    }
    .....
}

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is categories.size() value still -1 ?

yesss...