QDatabase error

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I dont know why, but i try to use a QDatabase for SQLite Queries. In an earlier Project everything went fine, but now i want to do a workaround to solve a problem from that earlier Project. Im trying to open and close the Database everytime if i do a Query. But i get an Error that the driver is not loaded and the database is not open. First i show my Code then the Error.

#include <QStandardPaths>
#include <QSqlQuery>
#include <QSqlError>
#include <QDir>
#include <QDebug>

gdms::Database::Database()
{
    //Init Create Tables
    dbTables();

}

void gdms::Database::instanceDb(const QString &name)
{
    QString appDataLocation = QStandardPaths::writableLocation(QStandardPaths::AppDataLocation);
    mDb = QSqlDatabase::addDatabase("QSQLITE", name);
    QDir dir(appDataLocation);
    if (!dir.exists())
        dir.mkpath(appDataLocation);
    appDataLocation.append(QString(QDir::separator()) + "gdms.sqlite");
    mDb.setDatabaseName(appDataLocation);
    if (!mDb.open())
        qDebug() << "Error open Database. " + mDb.lastError().text();
}

void gdms::Database::closeDb(const QString &name)
{
    if (mDb.isOpen())
    {
        mDb.close();
        mDb = QSqlDatabase();
    }
    mDb.removeDatabase(name);
}

void gdms::Database::dbTables()
{
    instanceDb("initdb");

    QSqlQuery query;
    query.prepare("CREATE TABLE IF NOT EXISTS treeview (id INTEGER PRIMARY KEY, index INTEGER, name TEXT)");
    if (!query.exec())
        qDebug() << "An Error occured during init/create Tables. " + query.lastError().text();

    closeDb("initdb");
}

and this i get when i run the app

QSqlQuery::prepare: database not open
"An Error occured during init/create Tables. Driver not loaded Driver not loaded"
QSqlQuery::prepare: database not open

Anyone knows what i do wrong? i really cant find a way to fix it. I cant create the Table in the Databasefile

the_

Just a guess:

QSqlQuery q; uses the default database connection.
As you have set a connection name, you do not have a default connection so you have to tell the query which database to use
use QSqlQuery q(mDb); instead.

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thanks. that helped me.