Returning QSqlQueryModel for TableView

Emre MUTLU

DB *db = new DB; 
QSqlQueryModel *model=new QSqlQueryModel;
model->setQuery(db->SelectData());
ui->tableView->setModel(model);
delete db;//closed and deleted db objects

db.cpp:

DB::~DB()//db destructor
{
database.close();
QSqlDatabase::removeDatabase("qt_sql_default_connection");//your dbname here
}

example class.cpp:

{
DB *db=new DB;
do something
delete db;
}

example class1.cpp:

DB *db=new DB;
do something
delete db;//we deleting db object and its call DB destructor

masa4

@Emre-MUTLU
constructor:

DB::DB(const QString &path)
{
    m_db = QSqlDatabase::addDatabase("QSQLITE");
    m_db.setDatabaseName(path);

    if(!m_db.open())
        qDebug() << "Error: connection with database failed";
    else
        qDebug() << "Database: connection ok";
    qDebug() << m_db.lastError().text();
}

destructor:

DB::~DB()
{
    m_db.close();
    m_db.removeDatabase("mypath"); //value of "path"  variable in constructor
}

and i got:

Database: connection ok
""
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
Database: connection ok
""
""
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.
Database: connection ok
""
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
Database: connection ok
""
QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.
Database: connection ok
""
""

Every time new db oject instantiated with DB db = new DB(); i got these errors. Im using delete db like you recommended.

Emre MUTLU

DB::~DB()
{
    m_db.close();
    m_db.removeDatabase("qt_sql_default_connection"); 
}

jsulm

@masa4 There is really no need to add same database connection again and again. Do it once and use it. That is why I pointed you to the documentation. You can get existing database at any time in any place of your application using https://doc.qt.io/qt-6/qsqldatabase.html#database

To analyse the errors you posted: how many DB instances do you have at the same time? Because each of them will try to add same database...

Emre MUTLU

@masa4

DB::~DB()
{
    m_db.close();
    m_db.removeDatabase(mypath); your path is string value already so you dont need to use(");
}`

JonB

@Emre-MUTLU , @masa4
Up to you, but as @jsulm/documentation states you really should not have a QSqlDatabase m_db member variable in a class. If you need to access the QSqlDatabase instance use static QSqlDatabase QSqlDatabase::database().

Emre MUTLU

@JonB

DB::~DB()
{
    m_db.close();
   m_db=QSqlDatabase();
    m_db.removeDatabase(mypath); your path is string value already so you dont need to use(");
}

like that?

JonB

@Emre-MUTLU
No! You have a QSqlDatabase m_db member variable in your DB class. Start by getting rid of that variable and then make your code work! Remember that instead you can re-access the current database instance via QSqlDatabase::database() if you need it. You may have QSqlDatabase db variables local to methods if you wish, but no class member variable which persists for the lifetime of the class.

masa4

@jsulm @JonB So you guys mean change code to like:

DB::DB(const QString &path)
{
    QSqlDatabase::addDatabase("QSQLITE");
    QSqlDatabase.setDatabaseName(path); //this doesnt work, the method is not static.

    if(!QSqlDatabase::open()) //non static, not work
        qDebug() << "Error: connection with database failed";
    else
        qDebug() << "Database: connection ok";
    qDebug() << QSqlDatabase::lastError().text(); //non static, not work
}

I got errors from nonstatic functions in this way. And secondly if i try to call from another class( selectData() is method of DB class.)
otherclass.cpp:

//db = new DB();
//model->setQuery(db->selectData(ui->lineedit->text())); //normally its in this way
model->setQuery(QSqlDatabase::database().selectData(ui->lineedit->text())); //No member named 'selectData' in 'QSqlDatabase'

How should i modify my class actually?

jsulm

@masa4 said in Returning QSqlQueryModel for TableView:

So you guys mean change code to like

No, we only suggested to not to have the m_db member variable in your DB class. I don't know why you changed model->setQuery part.

masa4

@jsulm Ho okay i see, so I will continue to use

db = new DB();
some code
delete DB();

paradigm, right? And what can u say for other non static methods? Without m_db how will i use them? In destructor i cant use QSqlDatabase::close() without member variable too.

masa4

@Emre-MUTLU No no i mean i used same path in both sides.
constructor:

m_db.setDatabaseName(DB_PATH);

destructor

m_db.removeDatabase(DB_PATH);

I am passing same value to both but i keep getting this error every time new DB(); called i think:

QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

jsulm

@masa4 said in Returning QSqlQueryModel for TableView:

And what can u say for other non static methods? Without m_db how will i use them?

Which non static members do you mean? As already explained you can get the database instance you need by simply calling QSqlDatabase::database().

And with your current implementation you're still creating the connection everytime you create a DB instance. You should use https://doc.qt.io/qt-6/qsqldatabase.html#contains to check whether the connection already exist and only add the connection if it does not yet exist.

Emre MUTLU

@masa4
i think you db name defaultly is qt_sql_default_connection

QSqlDatabase::removeDatabase("qt_sql_default_connection");

try this

masa4

@jsulm You mean like this? :

DB::DB(const QString &path)
{
    if(!QSqlDatabase::contains(QSqlDatabase::database().connectionName())){
            QSqlDatabase::addDatabase("QSQLITE");
            QSqlDatabase::database().setDatabaseName(path);
    }

    if(!QSqlDatabase::database().open())
        qDebug() << "Error: connection with database failed";
    else
        qDebug() << "Database: connection ok";
    qDebug() << QSqlDatabase::database().lastError().text();
}
DB::~DB()
{
    QSqlDatabase::database().close();
}

masa4

@Emre-MUTLU Yes when i add this i dont see the error output, but i changed the code again.

JonB

@masa4
If it helps, you are welcome to use a local variable in a method to reference a database, just that you should not keep it around in a class member variable. You might have code looking like this (only a suggestion):

DB::DB(const QString &path)
{
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName(path);
    if (!db.open())
        qDebug() << db.lastError().text();
}

DB::~DB()
{
    bool valid = false;
    QString connectionName = "";
    {
        // the need for these enclosing parentheses for scope is explained at https://doc.qt.io/qt-6/qsqldatabase.html#removeDatabase
        // only because I am going to call `removeDatabase()` here
        QSqlDatabase db = QSqlDatabase::database();
        valid = db.isValid();
        if (valid)
        {
            connectionName = db.connectionName();
            if (db.isOpen())
                db.close();
        }
    }
    if (valid && !connectionName.isEmpty())
        QSqlDatabase::removeDatabase(connectionName);
}