Strange problem with mysql and QTableView.

[Datakim]

Hi everyone, a newbie at qt requesting a bit of assistance here.

My problem is that I am trying to connect to a mysql database, and then load data from a table into a QTableView. Unfortunately, I have run into some difficulties.

I have done all the necessary mysql bits such as compiling a plugin and such, and I believe I did things correctly since I have managed to create a connection. My problem is that while the code works in one specific circumstance, it does not in another.

My test program is composed of two files, main.cpp and mainwindow.cpp. I am using qtcreator to make this program. I have looked up on how to load data into QTableView and if I do the loading in the main.cpp, it works right. I get a window with the correct data.

However, if I try to do the EXACT same thing with the exact same code from mainwindow.cpp, it fails to work at all. I get no errors or such, the data simply fails to load.

Here is a sample of the code I am using to make a connection that works:

main.cpp:

#include <QtGui/QApplication>
#include "mainwindow.h"
 
void initializeModel(QSqlTableModel *model)
{
    model->setTable("TABLE");
    model->setEditStrategy(QSqlTableModel::OnManualSubmit);
    model->select();
}
 
 
QTableView *createView(const QString &title, QSqlTableModel *model)
{
    QTableView *view = new QTableView;
    view->setModel(model);
    view->setWindowTitle(title);
    return view;
}
 
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
 
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("HOSTNAME");
    db.setDatabaseName("DATABASENAME");
    db.setUserName("USERNAME");
    db.setPassword("PASSWORD");
 
    if (!db.open()) {
        qDebug() << db.lastError();
        return 1;
    }
 
    QSqlTableModel model;
 
    initializeModel(&model);
 
    QTableView *view1 = createView(QObject::tr("Table Model"), &model);
    view1->show();
 
//    MainWindow w;
//    w.show();
 
 
    return a.exec();
}

#include <QtGui/QApplication>
#include "mainwindow.h"

void initializeModel(QSqlTableModel *model)
{
    model->setTable("TABLE");
    model->setEditStrategy(QSqlTableModel::OnManualSubmit);
    model->select();
}


QTableView *createView(const QString &title, QSqlTableModel *model)
{
    QTableView *view = new QTableView;
    view->setModel(model);
    view->setWindowTitle(title);
    return view;
}


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);


    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("HOSTNAME");
    db.setDatabaseName("DATABASENAME");
    db.setUserName("USERNAME");
    db.setPassword("PASSWORD");

    if (!db.open()) {
        qDebug() << db.lastError();
        return 1;
    }

    QSqlTableModel model;

    initializeModel(&model);

    QTableView *view1 = createView(QObject::tr("Table Model"), &model);
    view1->show();

//    MainWindow w;
//    w.show();


    return a.exec();
}

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The code above works. However, if I comment the sql code here, and remove the comments that open the MainWindow, and put the same code there, things no longer work.

Mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
#include <QtGui>
#include <QtSql>
 
 
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void initializeModel(QSqlTableModel *model)
{
    model->setTable("TABLE");
    model->setEditStrategy(QSqlTableModel::OnManualSubmit);
    model->select();
}
 
QTableView *createView(const QString &title, QSqlTableModel *model)
{
    QTableView *view = new QTableView;
    view->setModel(model);
    view->setWindowTitle(title);
    return view;
}
 
 
void MainWindow::on_pushButton_clicked()
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("HOSTNAME");
    db.setDatabaseName("DATABASENAME");
    db.setUserName("USERNAME");
    db.setPassword("PASSWORD");
 
 
    if (!db.open()) {
        qDebug() << db.lastError();
    }
 
    QSqlTableModel model;
 
    initializeModel(&model);
 
    QTableView *view1 = createView(QObject::tr("Table Model"), &model);
    view1->show();
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QtGui>
#include <QtSql>


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

MainWindow::~MainWindow()
{
    delete ui;
}

void initializeModel(QSqlTableModel *model)
{
    model->setTable("TABLE");
    model->setEditStrategy(QSqlTableModel::OnManualSubmit);
    model->select();
}

QTableView *createView(const QString &title, QSqlTableModel *model)
{
    QTableView *view = new QTableView;
    view->setModel(model);
    view->setWindowTitle(title);
    return view;
}


void MainWindow::on_pushButton_clicked()
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
    db.setHostName("HOSTNAME");
    db.setDatabaseName("DATABASENAME");
    db.setUserName("USERNAME");
    db.setPassword("PASSWORD");


    if (!db.open()) {
        qDebug() << db.lastError();
    }

    QSqlTableModel model;
  
    initializeModel(&model);

    QTableView *view1 = createView(QObject::tr("Table Model"), &model);
    view1->show();
}

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The code is exactly the same, except it runs from a button click rather than automatically. The information (hostname,databasename,etc are correct).
I have no idea whats wrong. Any advice or ideas would be much appreciated.
Thanks

[ChrisW67]

The QSqlTableModel you create at line 49 is on the stack, goes out of scope at the end of that routine, and leaves the view without a model to display. Create it on the heap.

It works in the first example because the model stays in scope long fro the entire lifetime of the view.

[CroOm]

in other words, add the following line to mainwindow.h

QTableView *view1;
QSqlTableModel model;

and assign them in mainwindow.cpp just with the names view1 and model