Question

I have recently begun working with QT and I’m having a small issue. I cant seem to understand how QVariant works. I’ve been going through the help and looking online but it is just not sinking in. In my project i am trying to populate a combo box with a list of manufacturers from a database. I have opened the database and pulled out the entries but I rename them all manValue. Now I understand why this is happening, the problem is I do not understand how to properly use QVariant to get the result I want. Originally i thought “manValue” was going to be the identifier for the string that held the actual value from the database but instead it reads the value from the database and makes sure it is not null, and then renames it. I already tried making a string before I assign the QVariant with any properties and assigning it the text i received from the database, and then inserting that string where manValue is but still no luck. Any help would be greatly appreciated. Sorry, I know this is going to be simple I’m just a noob and the help docs often confuse me.

QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
db.setHostName("LOCALHOST\\TestERPServer");
db.setDatabaseName("TestERPConnection");

if (db.open())
{
    QMessageBox::information(this,"Connected","Connection to the Database was Established\n"
                             "\nStatus: Connected");

    QSqlQuery mfrQry;

    if (mfrQry.exec("SELECT * FROM erp_data.manufacturers;"))
    {
        if (mfrQry.value(1) == "")
        {
            QMessageBox::information(this,"No Connection","Nothing in the Manufacturer Database\n"
                                     "\nError: " + db.lastError().text());
        }
        else
        {
            while (mfrQry.next())
            {
                ui->mfrComboBox->addItem("manValue",QVariant(mfrQry.value(1)));
            }
        }
    }
    else
    {
        QMessageBox::information(this,"No Connection","Connection to the Manufacturer Database could not be Established\n"
                                 "\nError: " + db.lastError().text());
    }
}
else
{
    QMessageBox::information(this,"No Connection","Connection to the Database could not be Established\n"
                             "\nError: " + db.lastError().text());
}

Answer 1

The first problem in the provided code has to do with the way you manipulate the QSqlQuery.

Successfully executed SQL statements set the query’s state to active
so that isActive() returns true. Otherwise the query’s state is set to
inactive. In either case, when executing a new SQL statement, the
query is positioned on an invalid record. An active query must be
navigated to a valid record before values can be retrieved.

In order to move to a valid record you have to use one of the following:

• next()
• previous()
• first()
• last()
• seek()

Once you are on a valid record you have to use the value() function in order to take the column you want. The returned value is QVariant so you need to convert to the desired type using one of the many toSomething functions QVariant provides.

So in your case the code should look like this:

QSqlQuery mfrQry;

if (mfrQry.exec("SELECT * FROM erp_data.manufacturers;"))
{
   // This will loop through all records returned by the query
   while (mfrQry.next()) {
         //  mfrQry.value(COLID) returns a QVariant containing the data of the current 
         // record in column COLID. 
         // Using toString we convert it to String
         QString stringValue = mfrQry.value(COLID).toString();
         // Now handle the QString the way you want...
  }
}