QML/QSqlQueryModel program Crashed I don't understand why



  • 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?


  • Lifetime Qt Champion

    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.



  • 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?


  • Lifetime Qt Champion

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



  • @SGaist Thanks man, everything work fine :)


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