thanks a lot.

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

QSqlDatabase DB2= QSqlDatabase::addDatabase("QSQLITE", "Db2");
    DB2.setDatabaseName("/home/mypath/DB2.sqlite3");
    QSqlDatabase Dg = QSqlDatabase::addDatabase("QSQLITE", "g");
    Dg.setDatabaseName("/home/mypath/g.sqlite3");

    if (!Db2.open() || !Dg.open())
    { /*  etc etc .... */

when start the app ..... so need to call only one time .... usign reopenDB() call from mainwindows too? good to know. Is these?

thanks

QSqlDatabasePrivate::addDatabase: duplicate connection name 'Db2', old connection removed.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'g', old connection removed.

Do you call addDatabase somewhere else (not only in reopenDB())?

i try variuos scenario .... al least error become from that wrong code:

void MainWindow::showUploadDb(int pageIdx){


    QSqlDatabase::database("LavDb").close();  /* <- these the problem */

    closeDB();

/** my code */
}

after change these into these one:

void MainWindow::showUploadDb(int pageIdx){


    closeDB();

/** my code */
}

plus closeDB:

void MainWindow::closeDB(){
    stopChartF = true;  /* only a relais */ 
    QSqlDatabase::removeDatabase("Db2");
    QSqlDatabase::removeDatabase("g");  /* these is a therd db open for test pourpuse and hel me ... because no query on it at all */
    QTimer::singleShot(400, this, SLOT(reopenDB()));
}

plus reopenDB:

void MainWindow::reopenDB(){

    QSqlDatabase DB2= QSqlDatabase::addDatabase("QSQLITE", "Db2");
    DB2.setDatabaseName("/home/mypath/DB2.sqlite3");
    QSqlDatabase Dg = QSqlDatabase::addDatabase("QSQLITE", "g");
    Dg.setDatabaseName("/home/mypath/g.sqlite3");
    //qDebug() << "DB2 ... REOPENdb... problemi ..." << DB2.lastError();

    if (!Db2.open() || !Dg.open())
    {
        QMessageBox msgBoxD1B;
        msgBoxD1B.setStyleSheet("background-color:gray");
        msgBoxD1B.setIcon(QMessageBox::Critical);
        msgBoxD1B.setWindowTitle("ERROR D.B.");
        msgBoxD1B.setText("Impossibile connect to the world DB");
        msgBoxD1B.setInformativeText("restart");
        msgBoxD1B.setStandardButtons(QMessageBox::Ok);
        msgBoxD1B.setDefaultButton(QMessageBox::Ok);
        int retD1B = msgBoxD1B.exec();
        switch (retD1B) {
          case QMessageBox::Ok:

              break;
          default:
              // should never be reached
              break;
        }
    }
    else{

        Db2 = QSqlDatabase::database("Db2");
        Dg = QSqlDatabase::database("g");
        usleep(50000);
        stopChartF = false; /* only a relais */
        usleep(1000);

        sqlViewFupdate);
        sqlViewBupdate();
    }
}

all work as aspected and no error .... but these warning not understand at all ..... (remember "g" have not query at all, no table model or other view type at all, only db open ...)

QSqlDatabasePrivate::addDatabase: duplicate connection name 'Db2', old connection removed.
QSqlDatabasePrivate::addDatabase: duplicate connection name 'g', old connection removed.

why these .... In my opinion I close all correct, and reopen in correct way ... or not?

regards

If all you want to do is copy a file from client to server I do not see that you absolutely have to delete the QSqlTableModel prior to the operation. It might be advisable, but it may be sufficient to ensure that you have not previously done any updates to the file in the client.

Having said that, I think your code is a fair example of releasing the QSqlDatabase and then removing it anyway. But you must not then keep any QTableView around whose model is the QSqlTableModel.

[premise:
my intent is read a db2 and show some table in a server PC, after via TCP need to copy entire db2 from a machine CLIENT that is a producer of data, than read again db2 on server and show again some table. Unfortunately all machinery use SQlite db. ]

maybe better stategy was copy from client to server db2(client side) as db2backup(server side) .... and update table from db2backup to db2 .... in these way need to delete QSqlTableModel during update operation?

You're not right - the QSqlTableModel holds the QSqlQuery and therefore a database connection.

so seems to need to stop QSqlTableModel first .... than close db .... right?

Because until now I think query for tableview can be consider a "oneshot" use of db ....when data is printed db work is end so can cloese it. Is not right?

You're not right - the QSqlTableModel holds the QSqlQuery and therefore a database connection.

Plus all my query is inside a specific void call everytime for a specific pourpose .... so think example:

{
    QSqlDatabase db = QSqlDatabase::database("sales");
    QSqlQuery query("SELECT NAME, DOB FROM EMPLOYEES", db);
}
// Both "db" and "query" are destroyed because they are out of scope
QSqlDatabase::removeDatabase("sales"); // correct

is equal to:

void Mainwindows::sales()
{
    QSqlDatabase db = QSqlDatabase::database("sales");
    QSqlQuery query("SELECT NAME, DOB FROM EMPLOYEES", db);
}

void Mainwindows::closeDB(){
// Both "db" and "query" are destroyed because they are out of scope
QSqlDatabase::removeDatabase("sales"); // correct
 }

thanks a lot

You have the explanation on how to do it in the method documentation.

where are the problem that not see it??

You still have an instance of your db so it can't get closed.

QSqlDatabase::database("db2").close()
QSqlDatabase::removeDatabase("db2");