Hi all!

I have the following code:

Qt Code: 
  1. #ifndef QMYSQLCONNECTION_H
  2. #define QMYSQLCONNECTION_H
  3.  
  4. #include <QObject>
  5. #include <QSqlDatabase>
  6.  
  7. class QMysqlConnection: public QObject, protected QSqlDatabase
  8. {
  9.     Q_OBJECT
  10.  
  11.     friend class QMysqlTable;
  12.  
  13. public:
  14.     QMysqlConnection(const QString& host, const QString& user, const QString&
  15.             password, const QString database, QObject* = 0);
  16.  
  17.     bool isOpen(void) const;
  18.     void close(void);
  19. };
  20.  
  21. #endif // QMYSQLCONNECTION_H
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Qt Code: 
  1. #include <QDebug>
  2. #include <QSqlError>
  3.  
  4. #include "QMysqlConnection.h"
  5.  
  6. // Public functions
  7. QMysqlConnection::QMysqlConnection(const QString& host, const QString& user,
  8.         const QString& password, const QString database, QObject* parent):
  9.         QObject(parent)
  10. {
  11.     this->addDatabase("QMYSQL");
  12.  
  13.     this->setHostName(host);
  14.     this->setUserName(user);
  15.     this->setPassword(password);
  16.     this->setDatabaseName(database);
  17.  
  18.     this->open();
  19.  
  20.     qDebug() << "Last connection error = " << this->lastError();
  21. }
  22.  
  23. bool QMysqlConnection::isOpen(void) const
  24. {
  25.     return QSqlDatabase::isOpen();
  26. }
  27.  
  28. void QMysqlConnection::close(void)
  29. {
  30.     QSqlDatabase::close();
  31. }
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This returns me the following error:
QSqlError(-1, "Driver not loaded", "Driver not loaded")

But if I change this code
Qt Code: 
  1. this->addDatabase("QMYSQL");
  2.  
  3. this->setHostName(host);
  4. this->setUserName(user);
  5. this->setPassword(password);
  6. this->setDatabaseName(database);
  7.  
  8. this->open();
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to this code:
Qt Code: 
  1. QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
  2.  
  3. db.setHostName(host);
  4. db.setUserName(user);
  5. db.setPassword(password);
  6. db.setDatabaseName(database);
  7.  
  8. db.open();
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it works! Why this code works and the other code not?