multiple access to database qt sqlite3 - qt

Question

disclaimer: I'm sorry if the code is not neatly indented. know some may be irked at this.
I'm using qt5
I have two .cpp(register_user.cpp and parking.cpp) files that will connect to a database. I tried to connect register_user.cpp to the database and it works perfectly fine. It is connected and the sql commands work fine. Tut, I need to also connect another file which is the parking.cpp to the database. I connected parking.cpp to the database and then checked for the connection. it said that it is connected but the sql commands don't work.
How can I connect the second file to the database correctly?
login.h
#ifndef LOGIN_H
#define LOGIN_H
#include <QMainWindow>
#include <QtSql>
#include <QDebug>
#include <QFileInfo>
#include <main_interface.h>
namespace Ui {
class Login;
}
class Login : public QMainWindow
{
Q_OBJECT
public:
QSqlDatabase mydb;
void connClose()
{
mydb.close();
mydb.removeDatabase(QSqlDatabase::defaultConnection);
}
bool connOpen()
{
mydb=QSqlDatabase::addDatabase("QSQLITE");
mydb.setDatabaseName("C:/SQLite/sqlite-tools-win32-x86-3250200/IPark.db");
if(!mydb.open())
{
qDebug()<<("Failed to open database");
return false;
}
else
{
qDebug()<<("Connected. . .");
return true;
}
}
public:
explicit Login(QWidget *parent = 0);
~Login();
private slots:
void on_pushButton_clicked();
void on_pushButton_2_clicked();
private:
Ui::Login *ui;
};
#endif // LOGIN_H
register_user.h
#ifndef REGISTER_USER_H
#define REGISTER_USER_H
#include <QDialog>
#include "login.h"
namespace Ui {
class register_user;
}
class register_user : public QDialog
{
Q_OBJECT
public:
Login conn;
explicit register_user(QWidget *parent = 0);
~register_user();
private slots:
void on_pushButton_clicked();
private:
Ui::register_user *ui;
};
#endif // REGISTER_USER_H
register_user.cpp
#include <login.h>
#include "login.h"
#include "register_user.h"
#include "ui_register_user.h"
#include <QMessageBox>
register_user::register_user(QWidget *parent) :
QDialog(parent),
ui(new Ui::register_user)
{
ui->setupUi(this);
if(!conn.connOpen())
ui->label_reg->setText("Failed to open database");
else
ui->label_reg->setText("Connected. . .");
}
register_user::~register_user()
{
delete ui;
}
void register_user::on_pushButton_clicked()
{
Login conn;
QString username, plate_number, name;
username=ui->lineEdit_Username->text();
plate_number=ui->lineEdit_Plate_Number->text();
name=ui->lineEdit_Name->text();
QSqlQuery qry;
qry.prepare("insert into User(User_id, plate_number, name, spot_number, credit, order_id) values('"+username+"','"+plate_number+"','"+name+"',NULL, NULL, NULL)");
if(qry.exec()){
QMessageBox::critical(this, tr("SAVE"), tr("SAVED"));
}
conn.connClose();
}
parking.h
#ifndef PARKING_H
#define PARKING_H
#include <QDialog>
#include "login.h"
namespace Ui {
class parking;
}
class parking : public QDialog
{
Q_OBJECT
public:
Login conn;
explicit parking(QWidget *parent = 0);
~parking();
private slots:
void on_ParkSpace100_clicked();
void on_ParkSpace101_clicked();
void on_ParkSpace102_clicked();
void on_ParkSpace103_clicked();
private:
Ui::parking *ui;
};
#endif // PARKING_H
parking.cpp
#include <login.h>
#include "login.h"
#include "parking.h"
#include "ui_parking.h"
#include "reservation.h"
#include <QMessageBox>
int status = 0;
parking::parking(QWidget *parent):
QDialog(parent),
ui(new Ui::parking)
{
ui->setupUi(this);
Login conn;
if(!conn.connOpen())
ui->label->setText("Failed to open database");
else
ui->label->setText("Connected. . .");
}
parking::~parking()
{
delete ui;
}
void parking::on_ParkSpace100_clicked()
{
Login conn;
QSqlQuery qry;
qry.prepare("insert into User(User_id, plate_number, name, spot_number, credit, order_id) values('105,0123,'Amethyst',NULL, NULL, NULL)");
if(qry.exec()){
QMessageBox::warning(this, tr("SAVE"), tr("SAVED"));
}
}
void parking::on_ParkSpace101_clicked()
{
Login conn;
QSqlQuery qry;
conn.connOpen();
qry.prepare("select name from User where spot_number = 101");
bool value = qry.exec();
if(value == true)//true : then someone is on the spot
{
conn.connClose();
QMessageBox::warning(this, "Error", "Someone's already in the spot");
qDebug("Inside ParkSpace101");
}
hide();
Reservation b;
b.setModal(true);
b.setWindowTitle("Reservation Page");
b.exec();
}
void parking::on_ParkSpace102_clicked()
{
QMessageBox::warning(this,"WARNING", "Someone's already in the spot.");
}
void parking::on_ParkSpace103_clicked()
{
Login conn;
QSqlQuery qry;
conn.connOpen();
qry.prepare("select name from User where spot_number = 101");
bool value = qry.exec();
if(value == true)//true : then someone is on the spot
{
conn.connClose();
QMessageBox::warning(this, "Error", "Someone's already in the spot");
qDebug("Inside ParkSpace101");
}
hide();
Reservation b;
b.setModal(true);
b.setWindowTitle("Reservation Page");
b.exec();
}

Answer 1

You'll want to pass a unique connectionName into the second argument in QSqlDatabase::addDatabase() in the connOpen method of login.h. This will allow you to add a new database connection while keeping the old one runnning.
Thus the line with addDatabase() should resemble
mydb = QSqlDatabase::addDatabase("QSQLITE", your_connection_name_here);
Make sure that the connectionName is different for register_user and parking. My suggestion is to pass the connectionName as a parameter into the constructor of Login. Then store this into a member variable.
explicit Login(const QString &connectionName, QWidget *parent = 0);
And modify the QSqlDatabase functions to handle the member variable
mydb.removeDatabase(m_connectionName);
...
mydb = QSqlDatabase::addDatabase("QSQLITE", m_connectionName);
When you initiate Login in register_user and parking, you can pass in your own connection name.
Login conn("register_user_sqlite_connection");
If you dislike passing arguments into the constructor, an alternative is to have a setter function for the connectionName.
void setConnectionName(const QString &connectionName);
then call it right after you initiate Login in register_user and parking.
Login conn;
conn.setConnectionName("register_user_sqlite_connection");
Further Reading: http://doc.qt.io/qt-5/qsqldatabase.html#addDatabase
Edit:
Minimal Example of Why OP's Code Does Not Work
#include <QDebug>
#include <QSqlDatabase>
#include <QSqlQuery>
class Login
{
public:
QSqlDatabase mDatabase; // Bad practice: don't use as class member.
// Read more: http://doc.qt.io/qt-5/qsqldatabase.html#details
Login() {}
bool connOpen()
{
// Only the default connection is used.
mDatabase = QSqlDatabase::addDatabase("QSQLITE");
mDatabase.setDatabaseName("stackOverflow.db");
if(!mDatabase.open())
{
qDebug() << ("Failed to open database");
return false;
}
qDebug() << "Connected. . .";
return true;
}
QSqlQuery exec(const QString& query)
{
return mDatabase.exec(query);
}
};
class RegisterUser
{
public:
Login conn;
RegisterUser()
{
qDebug() << "Opening database from RegisterUser()";
bool ok = conn.connOpen();
if (ok) qDebug() << "Connection success!";
else qDebug() << "Connection error...";
}
};
class Parking
{
public:
Login conn;
Parking()
{
qDebug() << "Opening database from Parking()";
bool ok = conn.connOpen();
if (ok) qDebug() << "Connection success!";
else qDebug() << "Connection error...";
}
};
int main()
{
// Initialise with default connection. 😁😁😁
RegisterUser reg;
// Opening database from RegisterUser()
// Connected. . .
// Connection success!
qDebug() << reg.conn.exec("SELECT 1").executedQuery();
// "SELECT 1"
// STEALS the default connection!!! 😡😡😡
Parking par;
// Opening database from Parking()
// QSqlDatabasePrivate::removeDatabase: connection 'qt_sql_default_connection' is still in use, all queries will cease to work.
// QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
// Connected. . .
// Connection success!
// Query from RegisterUser failed because it's not connected!!! 😱😱😱
qDebug() << reg.conn.exec("SELECT 1").executedQuery();
// QSqlQuery::exec: database not open
// ""
qDebug() << par.conn.exec("SELECT 1").executedQuery();
// "SELECT 1"
return 0;
}