Understanding "Connecting to Databases"

[graciano]

Hi

I'm trying to better undestand how the connection to a mysal database works(http://qt-project.org/doc/qt-5.0/qts...onnecting.html).

Ther is something i'm missing about removing a connection i still miss.

#include <QSqlDatabase>
 
QSqlDatabase *connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QMYSQL", "connection1"));
    db->setHostName(newHostName);
    db->setDatabaseName(newDatabase);
    db->setUserName(newUserName);
    db->setPassword(newPassword);
    return db;
}
 
int main()
{
    //QCoreApplication a(argc, argv);
    QSqlDatabase *db;
    bool dbStatus = false;
    for(int i=0; i<2;i++)
    {
        if(dbStatus)
        {
            db->close();
            db->removeDatabase("connection1");
            dbStatus = false;
        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db->open();
           dbStatus = true;
        }
    }
}

#include <QSqlDatabase>

QSqlDatabase *connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QMYSQL", "connection1"));
    db->setHostName(newHostName);
    db->setDatabaseName(newDatabase);
    db->setUserName(newUserName);
    db->setPassword(newPassword);
    return db;
}

int main()
{
    //QCoreApplication a(argc, argv);
    QSqlDatabase *db;
    bool dbStatus = false;
    for(int i=0; i<2;i++)
    {
        if(dbStatus)
        {
            db->close();
            db->removeDatabase("connection1");
            dbStatus = false;
        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db->open();
           dbStatus = true;
        }
    }
}

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This code loops the connect/disconnect to database.
Why do i get the message every time i remove the database?
"QSqlDatabasePrivate::removeDatabase: connection 'connection1' is still in use, all queries will cease to work."

[wysota]

You are not deleting the connection object anywhere. First of all there is no reason to use the new operator to create an instance of QSqlDatabase. Second of all a call to removeDatabase() has to occur only if all QSqlDatabase objects pointing to that connection have been deleted (it is not enough to close the connection).

Therefore this is wrong:

void fun() {
  QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
  QSqlDatabase::removeDatabase("connection1");
}

void fun() {
  QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
  QSqlDatabase::removeDatabase("connection1");
}

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and this is correct:

void fun() {
  {
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
  }
  QSqlDatabase::removeDatabase("connection1");
}

void fun() {
  {
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
  }
  QSqlDatabase::removeDatabase("connection1");
}

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[graciano]

You are not deleting the connection object anywhere

Grrr ... i forgot

First of all there is no reason to use the new operator to create an instance of QSqlDatabase.

I was thinking of an on/off button in the UI.

Anyway ... this works fine:

#include <QSqlDatabase>
 
QSqlDatabase *connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QMYSQL", "connection1"));
    db->setHostName(newHostName);
    db->setDatabaseName(newDatabase);
    db->setUserName(newUserName);
    db->setPassword(newPassword);
    return db;
}
 
int main()
{
    QSqlDatabase *db;
    bool dbStatus = false;
    for(int i=0; i<6;i++)
    {
        if(dbStatus)
        {
            db->close();
            delete db;
            QSqlDatabase::removeDatabase("connection1");
            dbStatus = false;
 
        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db->open();
           dbStatus = true;
        }
    }
}

#include <QSqlDatabase>

QSqlDatabase *connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase *db = new QSqlDatabase(QSqlDatabase::addDatabase("QMYSQL", "connection1"));
    db->setHostName(newHostName);
    db->setDatabaseName(newDatabase);
    db->setUserName(newUserName);
    db->setPassword(newPassword);
    return db;
}

int main()
{
    QSqlDatabase *db;
    bool dbStatus = false;
    for(int i=0; i<6;i++)
    {
        if(dbStatus)
        {
            db->close();
            delete db;
            QSqlDatabase::removeDatabase("connection1");
            dbStatus = false;

        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db->open();
           dbStatus = true;
        }
    }
}

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[wysota]

I was thinking of an on/off button in the UI.

This doesn't mean you have to use the new operator:

#include <QSqlDatabase>
 
QSqlDatabase connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
    db.setHostName(newHostName);
    db.setDatabaseName(newDatabase);
    db.setUserName(newUserName);
    db.setPassword(newPassword);
    return db;
}
 
int main()
{
    QSqlDatabase db;
    bool dbStatus = false;
    for(int i=0; i<6;i++)
    {
        if(dbStatus)
        {
            db.close();
            db = QSqlDatabase();
            QSqlDatabase::removeDatabase("connection1");
            dbStatus = false;
 
        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db.open();
           dbStatus = true;
        }
    }
}

#include <QSqlDatabase>

QSqlDatabase connection(QString newHostName, QString newDatabase, QString newUserName, QString newPassword)
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL", "connection1");
    db.setHostName(newHostName);
    db.setDatabaseName(newDatabase);
    db.setUserName(newUserName);
    db.setPassword(newPassword);
    return db;
}

int main()
{
    QSqlDatabase db;
    bool dbStatus = false;
    for(int i=0; i<6;i++)
    {
        if(dbStatus)
        {
            db.close();
            db = QSqlDatabase();
            QSqlDatabase::removeDatabase("connection1");
            dbStatus = false;

        }
        else
        {
           db = connection("localhost", "db01", "user1", "123");
           db.open();
           dbStatus = true;
        }
    }
}

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