How to change the displayed text in a QSqlRelationalDelegate subclass using the Qt::DisplayRole

Mark81

I wrote this delegate in Qt 6.8.2:

#ifndef DELEGATEEVENTS_H
#define DELEGATEEVENTS_H

#include <QSqlRelationalDelegate>
#include <QComboBox>
#include <QSqlDatabase>
#include <QSqlTableModel>
#include <QSqlQuery>
#include <QSqlError>
#include <QDebug>

#include "mainwindow.h"
#include "formprotocols.h"

class DelegateEvents : public QSqlRelationalDelegate
{
    Q_OBJECT

public:
    DelegateEvents(QObject *parent = nullptr) : QSqlRelationalDelegate(parent)
    {
    }

    QWidget *createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const override
    {
        const QSqlRelationalTableModel *sqlModel = qobject_cast<const QSqlRelationalTableModel *>(index.model());

        QModelIndex idx = sqlModel->index(index.row(), static_cast<int>(FormProtocols::Columns::MODEL_COL_TYPE));
        QString text = sqlModel->data(idx, Qt::EditRole).toString();
        if (text.isEmpty()) return nullptr;

        MainWindow::EventTypes eventType;
        bool ok;
        int i = text.toInt(&ok);
        if (ok) eventType = static_cast<MainWindow::EventTypes>(i);
        else
        {
            if (text == "Alarm") eventType = MainWindow::EventTypes::Alarm;
            else if (text == "Session") eventType = MainWindow::EventTypes::Session;
            else return nullptr;
        }

        QComboBox *editor = new QComboBox(parent);
        editor->setFrame(false);

        QSqlDatabase db = QSqlDatabase::database("mydb");
        QSqlQuery query(db);

        switch (eventType)
        {
        case MainWindow::EventTypes::Alarm:
            query.exec("SELECT id, gb FROM alarms;");

            while (query.next())
            {
                int id = query.value("id").toInt();
                QString name = query.value("gb").toString();
                editor->addItem(name, id);
            }
            break;

        case MainWindow::EventTypes::Session:
            query.exec("SELECT id, gb FROM sessions;");

            while (query.next())
            {
                int id = query.value("id").toInt();
                QString name = query.value("gb").toString();
                editor->addItem(name, id);
            }
            break;
        }

        return editor;
    }

    void setEditorData(QWidget *editor, const QModelIndex &index) const override
    {
        int value = index.model()->data(index, Qt::EditRole).toInt();

        QComboBox *w = static_cast<QComboBox*>(editor);
        w->setCurrentIndex(w->findData(value));
    }

    void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override
    {
        QComboBox *w = static_cast<QComboBox*>(editor);
        int value = w->currentData().toInt();
        model->setData(index, "foo", Qt::DisplayRole); // <-- this is not shown
        model->setData(index, value, Qt::EditRole);
    }
};

#endif // DELEGATEEVENTS_H

The goal is to fit a specific request: basically the column where the delegate is installed will show the values from another table according to a specific value of the current FormProtocols::Columns::MODEL_COL_TYPE text.

When I create the editor I add the items in the QComboBox using the name as text and id as user-data. In this way when I have to set the editor value I can retrieve the correct item using the data property.

On the other side, when I need to write back the values to the model I set both the EditRole (the actual integer value that is committed to the database) and the DisplayRole that should printed in the cell of the QTableView. My current test implementation above should always print "foo". Instead it shows the EditRole, i.e. the id value.

QSqlRelationalDelegate does not reimplement the paint function, hence I checked the source code for the QStyledItemDelegate and it seems it should print the DisplayRole:

    value = modelRoleDataSpan.dataForRole(Qt::DisplayRole);
    if (value->isValid() && !value->isNull()) {
        option->features |= QStyleOptionViewItem::HasDisplay;
        option->text = displayText(*value, option->locale);
    }

In order to debug what's happening I added a dummy paint function:

void paint(QPainter *painter, const QStyleOptionViewItem &option, const QModelIndex &index) const override
{
    QSqlRelationalDelegate::paint(painter, option, index);
}

so I was able to place a breakpoint and go deeper to reach the QStyledItemDelegate level. But the debugger told me the actual value for the DisplayRole was 3 (the id in my test case).

Hence I tried to understand why and I changed the function like this:

void setModelData(QWidget *editor, QAbstractItemModel *model, const QModelIndex &index) const override
{
    QComboBox *w = static_cast<QComboBox*>(editor);
    int value = w->currentData().toInt();
    model->setData(index, "foo", Qt::DisplayRole);
    model->setData(index, value, Qt::EditRole);
    qDebug() << model->data(index, Qt::EditRole) << model->data(index, Qt::DisplayRole);
}

and the output was:

QVariant(qlonglong, 3) QVariant(qlonglong, 3)

so it does not store the DisplayRole value. Why?

Unfortunately I cannot debug the setData function: placing a breakpoint there will hang the whole screen, nothing reacts anymore. I had to kill gdb from SSH. Tried few times.

I checked the return value of the first setData() and it was false, but in the documentation I cannot find how to find out the reasons (like the lastError for queries).

JonB

@Mark81
I am not sure I follow all that you have done/said. However, the way things work with a QSqlRelation is that the data table stores the desired value (like an integer id or similar) as its EditRole and the delegate looks that up in the foreign table to find the display value from there. Presumably ignoring any DisplayRole it would normally use for a data table and doing its own look up instead.

Mark81

@JonB ok, I changed my class so now it is a subclass of QStyledItemDelegate but nothing has changed. The behavior is the same.

By the way, I tried to reimplement displayText, but since it does not provide an index (just the text to be printed) I cannot retrieve the associated data from the model.

JonB

@Mark81
I have to confess I don't know what you are trying to "fix". I don't know what you expect to "change". It may (well) be that I don't understand what you are saying, but I don't think you can change the behaviour of DisplayRole/display text.

Nor do I understand why you are trying to do so. The whole point of QSqlRelationalTableModel is just to allow columns of a source table to hold a foreign key into a different table, so that the value (typically an integer "id", and stored in EditRole) is looked up in a foreign table and another column there (typically some text) is used as the DisplayRole value in the source table instead of its own. And QSqlRelationalDelegate is just for editing the source table in this situation, so that e.g. a combobox of the text values from the foreign table can be offered to the user when editing the source table instead of dealing with the underlying id number.