Why this->model->SetLinearVel doesn't work?

edit

I am trying to give velocity to a model by using SetLinearVel for the model and apparently it doesn't seem to work. I looked up this tutorial: http://gazebosim.org/tutorials?tut=se... and they are doing model->GetLink("white_link_0")->SetLinearVel({0, 1, 0});. However, I am attempting to do this->model->SetLinearVel(ignition::math::Vector3d(2.0, 0.0, 0.0)); and it doesn't seem to work. I don't see my model moving. What is it that I am doing wrong?

The full code is given below:

namespace gazebo
{
    Sparkle::Sparkle()
    {

    }
    void Sparkle::Load(physics::ModelPtr _parent, sdf::ElementPtr _sdf)
    {
        ROS_INFO_STREAM("Load");
        this->model = _parent; /*Store the pointer to the model*/
        this->world = _parent->GetWorld();
        this->robotNamespace = "";
        this->physicsEngine = (this->world)->Physics(); 

        /*Get the update rate from sdf*/
        if (_sdf->HasElement("updateRate"))
        {
            this->updateRate = _sdf->GetElement("updateRate")->Get<double>();
        }
        if (_sdf->HasElement("robotNamespace"))
        {
            this->robotNamespace = _sdf->GetElement("robotNamespace")->Get<std::string>();
        }


        this->velTopic = this->robotNamespace + "/vel";

        //Start up ros_node
        int argc = 0;
        char** argv = NULL;
        ros::init(argc, argv, "sparkle_sim", ros::init_options::NoSigintHandler | ros::init_options::AnonymousName);
        rosnode_ = new ros::NodeHandle(this->robotNamespace);


        this->ros_spinner_thread_ = boost::thread( boost::bind( &Sparkle::SimROSThread, this ) );


    }

    void Sparkle::SimROSThread()
    {
        ROS_INFO_STREAM("SimROSThread,");
        ros::NodeHandle nodehandle;
        ros::Subscriber velsub = nodehandle.subscribe(this->velTopic, 1, &Sparkle::changeVel, this);
        ros::Rate loop_rate(10);

        while (this->rosnode_->ok())
        {
            ROS_INFO_STREAM("Looping");
            ros::spinOnce();
            loop_rate.sleep( );
        }
    }

    void Sparkle::changeVel(const geometry_msgs::Twist::ConstPtr& msg)
    {
        ROS_INFO_STREAM("ChangingVel");
        double x = msg->linear.x;

        ROS_INFO("Velocity to set is %f\n", x);

        this->model->SetLinearVel(ignition::math::Vector3d(x, 0.0, 0.0));
    }

    void Sparkle::changePos(const gazebo_msgs::ModelState::ConstPtr& msg)
    {
        gazebo_msgs::ModelState newMsgs;
        newMsgs.model_name = this->robotNamespace;
        newMsgs.pose.position = msg->pose.position;
        newMsgs.pose.orientation = msg->pose.orientation;
        this->modelPub.publish(newMsgs);
    }
   GZ_REGISTER_MODEL_PLUGIN(Sparkle)
}

Apart from the fact that tutorial is using old math library, what is the difference between these two methods and why my method doesn't seem to work?