When receive slot is called, the ui is not updated but when I close the window, the qDebug in "opening movielist" and case 0 is run. How do I solve this? I need it to update the ui as per the integer passed via signals.

For me it looks like there is a forever loop which locks the thread event loop.
The emitted signals are stored in event loop, but as the thread is lock in a forever loop, the associated slots are called when forever loop ends and event loop is running.

What are you doing in movie class?

#include "movielist.h"
#include "ui_movielist.h"
#include <QSqlDatabase>
#include <QSqlError>
#include <QSqlQueryModel>
#include <QSqlQuery>
#include <QDebug>
#include <QSqlRecord>
#include <QMessageBox>
#include "movie.h"

Movielist::Movielist(QWidget *parent) : QDialog(parent),
                                        ui(new Ui::Movielist)
{
    ui->setupUi(this);
    QSqlDatabase myfile;
    myfile = QSqlDatabase::addDatabase("QSQLITE");
    myfile.setDatabaseName("C:/Users/user/Documents/New folder/qtcode/movie.db");
    if (!myfile.open())
    {
        qDebug() << ("Failed to open the database");
    }
    else
    {
        qDebug() << ("Connected");
    }
    QSqlQueryModel *model = new QSqlQueryModel();
    QSqlQuery qry;

    
}

Movielist::~Movielist()
{
    delete ui;
}

void Movielist::on_tableView_activated(const QModelIndex &index)
{
    movie m;
    int c = index.column();
    QString temp = ui->tableView->model()->data(index).toString();
    qDebug() << temp;
    m.setModal(true);
    hide();
    m.exec();
}

void Movielist::receive(int a)
{

    qDebug()<<"opening movielist";

    switch (a)
    {
    case 0:
    {
        qDebug()<<"case 0";
        QSqlQueryModel *model = new QSqlQueryModel();
        QSqlQuery qry;
        qry.prepare("select Name, Date, Ratings, Genre from movie order by Ratings DESC");
       qDebug()<<"Case 0 ko query";
        if (qry.exec())
        {
            model->setQuery(qry);
            ui->label_2->setText("Top Rated Movies");
            ui->tableView->setModel(model);
            ui->tableView->resizeColumnsToContents();
            qDebug()<<"case 0 query executed";
        }
        break;
    }
    case 1:
    {
        QSqlQueryModel *model1 = new QSqlQueryModel();
        QSqlQuery qry1;
        qry1.prepare("select Name, Date, Ratings, Genre from movie where Genre = 'Romantic' order by Ratings DESC ");
        if (qry1.exec())
        {
            model1->setQuery(qry1);
            ui->label_2->setText("Top Romantic Movies");
            ui->tableView->setModel(model1);
            ui->tableView->resizeColumnsToContents();
        }
        break;
    }
    }
}

When receive slot is called, the ui is not updated but when I close the window, the qDebug in "opening movielist" and case 0 is run. How do I solve this? I need it to update the ui as per the integer passed via signals.

And why do you want to call the slot? Slot is called automatically as soon as the signal is emitted.

Okay, I understood this. But I want to know how I am going to call this slot?

Please, start to learn C++ basics:

• what are classes
• variable scope

If you define a local variable in a function, this variable is local to this function and can not be used in another function.

#include "lists.h"
#include "ui_lists.h"

lists::lists(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::lists)
{
    ui->setupUi(this);
    QString a;
//I should be calling the slot here??? If yes, how?
    qDebug()<<a;//just an example
}

lists::~lists()
{
    delete ui;
}

void lists::get_movie(QString m)
{
    a = m; // I
    ui->label->setText(m);
}

This says a is an undeclared identifier. How do I call this slot and where?
While calling, will it not ask for variable 'm'?

// In List class
void List::get_movie(QString m)
{
    a = m; // I assume a was defined as class member, but could also be a local variable
    ui->label->setText(m);
}

// In List class
void List::get_movie(QString m)
{
    ui->label->setText(m);
}