QSqlDatabase, removeDatabase

Dear All,

I come back to a very popular error-message. I tried out a lot. I RTFM. I have no clue how to avoid the following mesage if dataGrabber is destrouyed.

Quote:

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QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.

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Code:

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class dataGrabber : public QObject
{
    Q_OBJECT
public:
    explicit dataGrabber(QObject *parent = nullptr);
    ~dataGrabber();
 
private:
    QSqlDatabase dB;
    QSqlQuery *query;
 
public slots:
    bool openDatabase();
    bool closeDatabase();
}

---

Code:

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bool dataGrabber::openDatabase(){
 
    dB = QSqlDatabase::addDatabase("QODBC");
    dB.setHostName("localhost");
    dB.setDatabaseName("DRIVER={Microsoft Access Driver (*.mdb)};FIL={MS Access};"
                               "DSN='';DBQ=" + dbPath + ";");
 
    if(dB.open()){
        query = new QSqlQuery(dB); // open dB and prepare query
        prepareQuery();
        return true;
    {
    else{
        QMessageBox::warning(0, "Failure !", dB.lastError().text(), "OK");
        return false;
    }
 
    return true;
}

---

Code:

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bool dataGrabber::closeDatabase(){
 
    query->finish();
    query->clear();
 
    delete query;
 
    dB.close();
 
    return true;
}

---

Code:

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dataGrabber::~dataGrabber(){
 
    dB.removeDatabase("qt_sql_default_connection");
}

---

As always, every help highly appreciated.

Cheers and thanks.

See https://doc.qt.io/qt-5/qsqldatabase.html#removeDatabase - there is still a reference in your member 'QSqlDatabase dB;'

Thanks ChristianEhrlicher

Quote:

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there is still a reference in your member 'QSqlDatabase dB;'

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Yes, that's why it fails. But I do not get it. The tutorial (which I read) says to leave the function containing dB for the object to be destroyed and then call dB.removeDatabase(). How can I call something on a destroyed object?
Or am I completely mislead here?

Quote:

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Originally Posted by mustermann.klaus@gmx.de

How can I call something on a destroyed object?

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QSqlDatabase::removeDatabase() is a static method, just like QSqlDatabase::addDatabase().


So that would probably look similar to this

Code:

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const QString connectionName = db.connectionName();
db = QSqlDatabase();
QSqlDatabase::removeDatabase(connnectionName);

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Cheers,
_

Okay, but I know the connectionName.

Code:

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dB.removeDatabase("qt_sql_default_connection");

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I understand why but really stucked on the question from where to call that ominous removeDatabase()

Cheers, Lars

Simply ise the code from anda_skoa ...

If it's that easy...Now it looks like this:

Code:

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dataGrabber::~dataGrabber(){
 
    const QString connectionName = dB.connectionName();
    dB = QSqlDatabase();
    dB.removeDatabase(connectionName);
}

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Somehow I do not believe that I remove the right dB from the list. Do I?
The errormessage is gone. But dB is still a member of my class datagrabber, which I simply overwrite with an invalid object. I really do not understand, if this is the way, why I should do so.

greetingd from rainy Berlin, Lars

That' not the code anda_skoa wrote for you... and why it has to be done that way was explicitly written in the documentation I gave you the link to - db holds a reference which also needs to be removed *before* QSqlDatabase::removeDatabase() is called.

Quote:

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Originally Posted by mustermann.klaus@gmx.de

Somehow I do not believe that I remove the right dB from the list. Do I?

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Why would it not be the right one?

Aside from not calling the static method as usual with "ClassName::methodName" this is passing the connection name of the "db" object you have been using.
Which, I assume, is the one you would want to remove.

Quote:

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Originally Posted by mcanonic

But dB is still a member of my class datagrabber, which I simply overwrite with an invalid object.

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Since "dB" is not a pointer, the only way to make it drop its connection is by invalidating it through overwriting.

Cheers,
_

Hello everybody. Thanks for guiding me. To me it still seems confusing that an object has no method to end a connection to the world without beeing destroyed. But if so, I'll take it.

Code:

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dataGrabber::~dataGrabber(){
 
        const QString connectionName = dB.connectionName();
        dB = QSqlDatabase();
        QSqlDatabase::removeDatabase(connnectionName);
    }

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Works fine. Thank you.

Lars

I think QSqlDatabase is designed to be used in a more ad-hoc way, not as member variable perhaps.


Either way doing dB->close() seems cleaner to me?

Quote:

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Originally Posted by tuli

I think QSqlDatabase is designed to be used in a more ad-hoc way, not as member variable perhaps.


Either way doing dB->close() seems cleaner to me?

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I think so too (in the meantime). As you can see in my initial post: dB->close(); is what I did in first sight.

Cheers.