Problems with allocating QSqlTableModel on heap

I have a form with 2 PuchButtons and a TableView created using QTCreator. Now I want to populate TableView with data from SQL database using QSqlTableModel.
My question is: Why when i allocate QSqlTableModel in PushButton1Click everything is ok, and when I allacate it in MainWindow constructor the data from database doesn't show on TableView.
(First I click button 1 to open database, then I click button 2 to fetch data)
Here is declaration of mainwindow class:

Code:

class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
    QSqlTableModel* m;
 
private slots:
    bool PushButton1Click();
    bool PushButton2Click();
 
};

Here is code which works:

Code:

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent), ui(new Ui::MainWindow)
{
     ui->setupUi(this);
     // m = new QSqlTableModel(this);       // NOT OK !!!!!!!!
}
 
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
 
bool MainWindow::PushButton1Click()
  {
  QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
  db.setHostName("localhost");
  db.setDatabaseName("axis");
  db.setUserName("root");
  db.setPassword("kosa1980");
  bool ret = db.open();
 
  m = new QSqlTableModel(this);         // OK !!!!!!!!
  return ret;
  }
 
bool MainWindow::PushButton2Click()
{
     m->setTable("wagi");
     m->setEditStrategy(QSqlTableModel::OnManualSubmit);
     m->select();
     m->removeColumn(0);
     m->setHeaderData(0, Qt::Horizontal, QObject::tr("numer_seryjny"));
     m->setHeaderData(1, Qt::Horizontal, QObject::tr("typ"));
 
     ui->tableView->setModel(m);
 
     return true;
}

Here is code which doesn't work:

Code:

#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent), ui(new Ui::MainWindow)
{
     ui->setupUi(this);
     m = new QSqlTableModel(this);       // NOT OK !!!!!!!!
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
bool MainWindow::PushButton1Click()
  {
  QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
  db.setHostName("localhost");
  db.setDatabaseName("axis");
  db.setUserName("root");
  db.setPassword("kosa1980");
  bool ret = db.open();
 
  //m = new QSqlTableModel(this);         // OK !!!!!!!!
  return ret;
  }
 
bool MainWindow::PushButton2Click()
{
     m->setTable("wagi");
     m->setEditStrategy(QSqlTableModel::OnManualSubmit);
     m->select();
     m->removeColumn(0);
     m->setHeaderData(0, Qt::Horizontal, QObject::tr("numer_seryjny"));
     m->setHeaderData(1, Qt::Horizontal, QObject::tr("typ"));
 
     ui->tableView->setModel(m);         
 
     return true;
}

Why second version doesn't work?