QT5.9.1 connect MySQL5.7.18

[LuAria]

I have finally used OT to connect MySQL successfully yesterday.
However, when I ran the project, it wraned me that

QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

I used

this->db = QSqlDatabase::addDatabase("QMYSQL");
this->db.setHostName("127.0.0.1");
this->db.setPort(3306);
this->db.setUserName("root");
this->db.setPassword("lulu168168");
this->db.setDatabaseName("student");

in many places.
Is that led to the warning?
Thanks you.

[LuAria]

What's more, I never used the sentence removeDatabase…

[Slane]

you need to create a deeper scoop
like :

// deeper scoop
{
    db = QSqlDatabase::addDatabase("QMYSQL","ConnectionName");
    db.setHostName("127.0.0.1");
    db.setPort(3306);
    db.setUserName("root");
    db.setPassword("lulu168168");
    db.setDatabaseName("student");
}
QSqlDatabase::removeDatabase("ConnectionName");

and it's may work

[jsulm]

@LuAria said in QT5.9.1 connect MySQL5.7.18:

in many places

Why? There is even no need to have this db variable. Just use the default connection and get it when needed using

QSqlDatabase db = QSqlDatabase::database();

And set up the database only once.
Currently you create many connections which is just waste of resources.
See http://doc.qt.io/qt-5/qsqldatabase.html

[LuAria]

@Slane Thanks, I will have a try.

[LuAria]

@jsulm Sorry it's my mistake. Maybe I cannot express myself very well.
I wrote many cpps on one project. And I wrote that code on even every cpp.
So I meant whether I wrote that code so many times led to the warning tip.

[jsulm]

@LuAria Yes, this is what I mean: there is no need to initialize same database connection several times in different places (different cpp files). Do it once and then get the connection where you need it like I shown.
The warnings you get are most probably caused by this problem: you have many active connections.

[LuAria]

@jsulm Thanks.
I think I get your point.
But how can I do it once and get the connection.(Is there a special code to get the connection?)
Should I define a global variable？
The other hand, should I use ~.close() and QSqlDatabase::database()
after using the database. I haven't use that two codes before?
Thank you very much!

[jsulm]

@LuAria Did you read the link I posted?
I even copy/pasted the line to get the connection:

QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
db.setHostName("acidalia");
db.setDatabaseName("customdb");
db.setUserName("mojito");
db.setPassword("J0a1m8");
bool ok = db.open();

// Then somewhere else, where you need to use the connection
QSqlDatabase db = QSqlDatabase::database();

No need for any global variable!
Where to initialize the connection and where to close it depends on your design. You can do it in main:

int main()
{
    QApplication app(...);
    QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
    db.setHostName("acidalia");
    db.setDatabaseName("customdb");
    db.setUserName("mojito");
    db.setPassword("J0a1m8");
    bool ok = db.open();
    MainWindow *mainWindow = new MainWindow();
    mainWindow->show();
    bool exitStatus = app.exec();
    // Here close the connection
    return exitStatus;
}

[LuAria]

@jsulm
I mean should I initialize same database connection once on one cpp, and then when I need using, I can only use the code
QSqlDatabase db = QSqlDatabase::database("QMYSQL", "connectionName");
to operate on the database?

[jsulm]

@LuAria Yes, this is what I'm saying.
To be sure: you do not have to initialize the connection in every cpp file! Just do it ONCE in your project.
You should really read the documentation which I mentioned before - it actually describes this.

[LuAria]

@jsulm Thanks a lot!
Sorry for my miss on the link.
I will try it.