QML/QSqlQueryModel program Crashed I don't understand why

[alirezafour]

Hi everyone
I'm new in Qt/C++ and QML. I was trying to Connect SQLite Database to QML with C++ QSqlQueryModel and QSqlDatabase. I found this and Used this to wrote my Program and this is my code:

//database.h

#include <QtSql/QSqlDatabase>
#include <QSqlQuery>
#include <QFileInfo>

class Database
{
public:
    Database();
    ~Database();
    void open();

private:
    QSqlDatabase *db;
};

//database.cpp
#include "database.h"
#include <QDebug>

Database::Database()
{
}

Database::~Database()
{
    db->close();
}

void Database::open()
{
    QFileInfo file("database.db");
    *db = QSqlDatabase::addDatabase("QSQLITE");
    db->setDatabaseName("database.db");
    db->setUserName("database");
    db->setPassword("database");
    if(file.exists())
    {
        db->open();
        return;
    }
    else
    {
        db->open();
        QSqlQuery query;
        query.exec("CREATE TABLE person ("
                   "id INT PRIMARY KEY,"
                   "name TEXT NOT NULL,"
                   "phone TEXT NOT NULL UNIQUE);");
        query.exec("INSERT INTO person(id, name, phone) "
                   "VALUES(1, 'Paul', '0123434' )");
        query.exec("INSERT INTO person(id, name, phone) "
                   "VALUES(2, 'Alex', '0765333' )");
        query.exec("INSERT INTO person(id, name, phone) "
                   "VALUES(3, 'Sara', '56753' )");
        query.exec("INSERT INTO person(id, name, phone) "
                   "VALUES(4, 'Mindcraft', '345755' )");

    }
}

//dataModel.h
#include <QSqlQueryModel>

class dataModel : public QSqlQueryModel
{
    Q_OBJECT

public:
    explicit dataModel(QObject *parent = 0);
    void setQuery(const QString &query, const QSqlDatabase &db = QSqlDatabase());
    void setQuery(const QSqlQuery &query);
    QVariant data(const QModelIndex &index, int role) const;
    QHash<int, QByteArray> roleNames() const {	return m_roleNames;	}

private:
    void generateRoleNames();
    QHash<int, QByteArray> m_roleNames;
};

//dataModel.cpp
#include "datamodel.h"

#include <QSqlRecord>
#include <QSqlField>


dataModel::dataModel(QObject *parent) :
    QSqlQueryModel(parent)
{

}

void dataModel::setQuery(const QString &query, const QSqlDatabase &db)
{
    QSqlQueryModel::setQuery(query, db);
    generateRoleNames();
}

void dataModel::setQuery(const QSqlQuery &query)
{
    QSqlQueryModel::setQuery(query);
    generateRoleNames();
}

QVariant dataModel::data(const QModelIndex &index, int role) const
{
    QVariant value;

    if(role < Qt::UserRole) {
        value = QSqlQueryModel::data(index, role);
    }
    else {
        int columnIdx = role - Qt::UserRole - 1;
        QModelIndex modelIndex = this->index(index.row(), columnIdx);
        value = QSqlQueryModel::data(modelIndex, Qt::DisplayRole);
    }
    return value;
}

void dataModel::generateRoleNames()
{
    m_roleNames.clear();
    for( int i = 0; i < record().count(); i ++) {
        m_roleNames.insert(Qt::UserRole + i + 1, record().fieldName(i).toUtf8());
    }
}

//main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QQmlComponent>
#include "model/datamodel.h"
#include "database/database.h"

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

    Database dbase;
    dbase.open();

    dataModel *model = new dataModel();
    model->setQuery("select  * from person");

    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    QQmlContext *context = new QQmlContext(engine.rootContext());
    context->setContextProperty("myModel", model);

//    QQmlComponent component(&engine);
//    component.setData("import QtQuick 2.0\nListView { model: myModel }", QUrl());
//    QObject *window = component.create(context);

    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();
}

//main.qml
import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
    id: root
    visible: true
    width: 640
    height: 480
    title: qsTr("Test QML Database Model")


    ListView {
        id: list_view
        anchors.centerIn: parent
        width: root/2
        height: root/2
        model: myModel
        delegate: Text {
            text: id + ": " + name + " " + phone
        }
    }
}

It build fine but whenever I run the program I got this The program has unexpectedly finished. and Crashed.

What is the problem?

[SGaist]

Hi and welcome to devnet,

First thing to do: create your QApplication object before anything else. It initializes Qt's internals.
You are dereferencing a pointer that you didn't allocate in your Database class.
Also, please re-read the QSqlDatabase documentation on how to handle the your database connection object.

[alirezafour]

thank you

I fix the database and model problem and they work fine but I still have problem to connect them to the QML

QQmlApplicationEngine engine;
    QQmlContext *context = new QQmlContext(engine.rootContext());
    context->setContextProperty("myModel", model);
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

this part of my code doesn't work as I expect.
the QQmlApplicationEngine work fine and show my qml file but the QQmlContext doesn't work and I got qrc:/main.qml:25: ReferenceError: myModel is not defined error message.
I was trying to debug it but debugger not show any data of QML or what I want to show.
I think I should call engine source before the setContextProperty but I try different way and It hasn't worked yet.
Can you tell me what I missed or what I need to learn to fix it?

[SGaist]

Why are you creating a new QQmlContext ? Just use the one the engine gives you.

[alirezafour]

@SGaist Thanks man, everything work fine :)