QSqlDatabase::select() fetches no data

[MasterQ]

Hi,

in my small QWidget application I acess a SQlite Database to fill a QTableView.

The SqlTableModel is created in a factory class as follow:

...

class DocumentsModel : QObject
{
    Q_OBJECT

    public:
    DocumentsModel();
    ~DocumentsModel();
    QSqlTableModel *ModelFactory();
    QSqlDatabase Database();
    void Open();
    void Close();
    void Select();
    void Dispose();

    private:
    QSqlTableModel *_model = nullptr;
    QSqlDatabase _db;
    QString const _driver = "QSQLITE";
    QString const _connection = "documents";
    QString _pathtodatabase = "PathToDataBase";
    QString const _tablename = "Dokumente";
};

...

with

QSqlTableModel *DocumentsModel::ModelFactory()
{
    if (_model == nullptr) {
        _model = new QSqlTableModel(this, Database());
        _model->setTable(_tablename);
    }
    return _model;
}

QSqlDatabase DocumentsModel::Database()
{
        _db = QSqlDatabase::addDatabase(_driver, _connection);
        auto connectionstring
            = DependencyProvider::GetSingleton<QSettings>()
                  ->value(_pathtodatabase, "/home/.../devel.db")
                  .toString();
        _db.setDatabaseName(connectionstring);
        _db.open(); // <---- open here
    return _db;
}

void DocumentsModel::Open()
{
    if (!_db.isOpen())
        _db.open();
}

void DocumentsModel::Close()
{
    if (_db.isOpen())
        _db.close();
}

void DocumentsModel::Select()
{
    if (_db.isOpen())
        _model->select();
}

this class is invoked by a QWidget elsewhere (different library):

QWidget * DocumentsView::DocumentsViewNew(){
...
    auto tableview = new QTableView();
    auto documentsmodel = new DocumentsModel();
    tableview->setModel(documentsmodel->ModelFactory());

    documentsmodel->Open();  // <----- or open here
    documentsmodel->Select();
...
}

The behaviour depends on where I open the database.

If the dabase is opened in the factory methode, the table is filled accordingly. If I open the database in the other QWidget which holds the TableView the table remains empty.

I do not understand the difference

The debugger shows, that in both cases no errors occur, all is looking fine except, that no data are fetched in the second case.

But why?

Greets

Joachim

[SGaist]

@MasterQ Hi,

A QSqlTableModel is a QSqlQueryModel which uses QSqlQuery.
When you call setTable, a query is done to retrieve the table's field information, thus the connection must be opened.

[Christian Ehrlicher]

You must open the database before creating a query.

[Pl45m4]

@MasterQ said in QSqlDatabase::select() fetches no data:

private:
       QSqlDatabase _db;

Better don't keep the database as member variable.
You can access the current opened databse with QSqlDatabase::database() any time, but having a database member might crash on shutdown when not done properly or lead to unwanted behavior.

[JonB]

@MasterQ said in QSqlDatabase::select() fetches no data:

except, that no data are fetched in the second case.

You have:

void DocumentsModel::Select()
{
    if (_db.isOpen())
        _model->select();
}

If you are debugging/asking can you please put in the necessary else or Q_ASSERT() or qDebug() to tell us whether the database is open and nothing is returned or whether this silently does nothing as the database is (presumably unexpectedly) not open? And select() returns a bool, please test it.

[MasterQ]

@JonB said in QSqlDatabase::select() fetches no data:

If you are debugging/asking can you please put in the necessary else or Q_ASSERT() or qDebug() to tell us whether the database is open and nothing is returned or whether this silently does nothing as the database is (presumably unexpectedly) not open? And select() returns a bool, please test it.

the database is open and select returns false

void DocumentsModel::Select()
{
    if (_db.isOpen())
        qDebug() << _model->select();
}

09:17:12: Debugge /home/joachim/...
false

[Christian Ehrlicher]

Again: do you create the model before opening the db?

[MasterQ]

@Christian-Ehrlicher said in QSqlDatabase::select() fetches no data:

Again: do you create the model before opening the db?

I just recognized that in one case the Model is created with an open db, in the other with a non-open db.

Must the db be open when the model is created?

[JonB]

@MasterQ said in QSqlDatabase::select() fetches no data:

Must the db be open when the model is created?

I guess so since that is what @Christian-Ehrlicher is asking you. If not I wonder how/when/why the model would get linked to the actual database/table when the database is opened at a later date. But if you want to know whether it would lead to the behaviour you see why don't you test in small program?

the database is open and select returns false

So, please, without my having to ask, if select() returns false then look at what is in QSqlQueryModel::lastError() and/or QSqlDatabase::lastError(). Production code should always include such error checking/reporting --- you never know when/why things might go wrong --- and development/debugging/problem code even more so.

[Christian Ehrlicher]

@MasterQ said in QSqlDatabase::select() fetches no data:

Must the db be open when the model is created?

I already told you twice that this is the case...

[MasterQ]

@Christian-Ehrlicher

your post was misleading for me.

you said the the db must be opened when a query is placed.

I read this as "an own query". To create a model is a different issue and I am wondering if it is mentioned somewhere in the documentation that for model creation the database must be explicitely opened before.

I expected that the db is opened internally while the model is created and immediately closed when it is done.

thanks for pointing out.

[SGaist]

@MasterQ Hi,

A QSqlTableModel is a QSqlQueryModel which uses QSqlQuery.
When you call setTable, a query is done to retrieve the table's field information, thus the connection must be opened.