QSqlDatbase is closed before the query finishes - c++

Question

I am trying to implement a database object for my application, but it became a nightmare when I started using multiple connections. Below, you can see my database C++ class code:
// here are the declarations
QString server_addr;
QString username;
QString password;
QString database_name;
QSqlDatabase connection;
QString error;
QString connectionName;
QSqlQuery m_query;
// and here are the definitions
database::database(QString connectionName) {
preferences p; p.read();
QString iConnectionName = (connectionName == "") ? default_connection_name : connectionName;
this->connectionName = iConnectionName;
if (QSqlDatabase::contains(iConnectionName))
this->connection = QSqlDatabase::database(iConnectionName);
else this->connection = QSqlDatabase::addDatabase("QMYSQL", iConnectionName);
this->connection.setHostName(p.database->server_addr);
this->connection.setUserName(p.database->username);
this->connection.setPassword(p.database->password);
this->connection.setDatabaseName(p.database->database_name);
this->connection.setPort(p.database->serverPort);
if (!connection.open())
{
this->error = this->connection.lastError().text();
}
else this->error = "";
}
QSqlQuery database::query(QString query_text) {
this->m_query = QSqlQuery(query_text, this->connection);
this->m_query.exec();
return m_query;
}
database::~database() {
if (!this->m_query.isActive()) QSqlDatabase::removeDatabase(this->connectionName);
qDebug() << "database object destroyed\n";
}
The problem occurs in a another class (that uses the database):
databaseAdaptor::databaseAdaptor() {
this->db = database();
// other construction operations
}
void databaseAdaptor::fetch() {
QSqlQuery q = db.query("SELECT * FROM `activities`");
qDebug() << "Rows count: " << q.numRowsAffected();
}
It worked some versions in the past, but for some reason, now the output from qDebug() is
Rows count: -1 // and it should be 2 for my current version of database.
In the Qt Documentation, it is said that this class' members are thread-safe. Could this be the problem? I mean, could the thread end before the queries finish their execution?
If so, how can I keep the connection open until all the queries finished their execution?
(I know it's much to read, but I am sure that other might have this problem at some point, so please take your time and read it). Thank you!