Not able to push a value from Qt to QML

Question

I am trying to read a value from mysql db through Qt and push the value to QML(specifically to a CircularGauge dial). I am successfully able to read the db value and print it to console in Qt. When I try to expose the value to QML, it is not getting reflected in QML dashboard. The following are my files:-

qml.h

#ifndef QML_H
#define QML_H

#include <QObject>
#include <QVariant>
#include <QDebug>
#include <QTime>
#include <qquickimageprovider.h>
#include <QImage>
#include <QString>

/*------------------------------SPEED------------------------------*/

class Speed : public QObject
{
    Q_OBJECT
    Q_PROPERTY(double velocity MEMBER m_velocity NOTIFY velocityChanged)

public:
    Speed(QObject* parent = nullptr);
    virtual ~Speed() {}

    const double MPS2KNOTS = 1.94;

    void velocityCallback();

private slots:
    void updateVelocity(double x);

signals:
    void velocityChanged();
    void receivedVelocity(double x);

private:
    double m_velocity = 0.0;


};

#endif // QML_H

qml.cpp

#include <QSqlDatabase>
#include <QSqlQueryModel>
#include <QSqlQuery>
#include <QDebug>
#include <qml.h>


Speed::Speed(QObject*)
{
    connect(this, SIGNAL(receivedVelocity(double)), this, SLOT(updateVelocity(double)));

}

void Speed::velocityCallback()
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
        db.setHostName("localhost");
        db.setDatabaseName("sim_speed");
        db.setUserName("administrator");
        db.setPassword("test");
        bool ok = db.open();

        double x;
        QSqlQuery query;
        //while (ok) {
            query.prepare("SELECT speed FROM speed");
            query.exec();
            if (query.next()) {
             x= query.value(0).toDouble();
            }
            qDebug() << "Value is";
            qDebug() << ok;
            qDebug() << x;
            receivedVelocity(x);
        //}

}

void Speed::updateVelocity(double x)
{

        m_velocity = x;
        emit velocityChanged();
        qDebug() << m_velocity;


}

main.cpp

#include <functional>

#include <QCoreApplication>
#include <QObject>
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QUrl>
#include <QString>
#include <QQuickWindow>
#include <QtQml>
#include <QtConcurrent/QtConcurrent>
#include <QFuture>
#include <QFutureWatcher>
#include <qml.h>



#include <QQmlProperty>
#include <thread>         // std::thread
#include <QDebug>
#include <QCoreApplication>
#include <QTextStream>
#include <QFile>

using namespace std;

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

    //std::thread dbret(velocityCallback());


    QGuiApplication app(argc, argv);

    Speed speedqml(&app);

    //QObject::connect(&app, &QCoreApplication::aboutToQuit, [](){});





    QQmlApplicationEngine engine(&app);
    engine.rootContext()->setContextProperty("velocity", &speedqml);

    Speed sp;
    sp.velocityCallback();

    engine.load(QUrl("qrc:/main.qml"));
   // std::thread dbret(mainn);
    //dbret.join();




    return app.exec();
}

main.qml

import QtMultimedia 5.8
import QtQuick 2.2
import QtQuick.Window 2.1
import QtQuick.Controls 1.4
import QtQuick.Controls.Styles 1.4
import QtQuick.Extras 1.4
import QtSensors 5.10
//import Ros 1.0




Window {

    visible: true

    width: 1920
    height: 1080


    title: qsTr("Gazebo")





        Rectangle {
            id: speedometer
            x: 10
            y: 10
            width: 150
            height:150
            radius: width*0.5
            opacity: 0.5
            color: "black"



           CircularGauge {
               id:gauge



               value: velocity.velocity
               maximumValue : 10
               x:10
               y:10
               width:150
               height:150
               anchors.centerIn: parent
               style: CircularGaugeStyle {
                   labelStepSize: 0.5
                   tickmarkStepSize: 0.5
               }

               Text {
                   text: "Speed:"+velocity.velocity+"m/s"
                   anchors.top:parent.bottom
                   anchors.horizontalCenter: parent.horizontalCenter
               }    
               Behavior on value {
                   NumberAnimation {
                       duration: 1000
                   }
               }
               MouseArea {
                   anchors.fill: parent
                   onClicked: {
//                       tachometer.visible = false
//                       speedometer.visible = false
//                       compassdial.visible = false
//                       altimeterback.visible = false
//                       ammeter.visible = false
//                       voltmeter.visible = false
//                       throttlegauge.visible = false
//                       dashboard.visible = true
                   }

           }
           }

           }
}