QComboBox and QSqlRelationalDelegate doesn't allow to choose element with index over 255

[BrokenVoodooDoll]

I work with QSqlRelationalTableModel and QSortFilterProxyModel. I made a custom delegate to work with the relational table model through the proxy model. But my relation tables contain a lot of records (more than 256), therefore combo boxes of the delegate contain a lot of rows. When I try to select an item from a combo box that has an index exceeding 256, the current index of the combo box just doesn't change. But if the index's row is lower than 256, it works. I tried it either with QSqlRelationalDelegate or with my custom "SqlRelationalProxyDelegate", nothing works in a proper way. I even tried to force the combo box's model to be fully fetched. I used a debugger and found out that setEditorData and setModelData methods of the delegate even aren't called when I select an item with a row value more than 256.
It worth mentioning that if I populate a combo box with QSqlQueryModel with the same tables, it works correctly. But I want to use delegate but not this crutch.
My code:
sqlrelationalproxydelegate.h

#include <QSqlRelationalDelegate>

class SqlRelationalProxyDelegate : public QSqlRelationalDelegate
{
    Q_OBJECT
public:
    SqlRelationalProxyDelegate(QObject* parent = nullptr);

    QWidget* createEditor(QWidget *parent,
                          const QStyleOptionViewItem &option,
                          const QModelIndex &index) const override;
    void setModelData(QWidget* editor,
                      QAbstractItemModel* model,
                      const QModelIndex& index) const override;
    void setEditorData(QWidget* editor, const QModelIndex& index) const override;
};

sqlrelationalproxydelegate.cpp

#include "sqlrelationalproxydelegate.h"

#include <QSortFilterProxyModel>
#include <QDebug>

SqlRelationalProxyDelegate::SqlRelationalProxyDelegate(QObject* parent)
    : QSqlRelationalDelegate(parent)
{}

QWidget *SqlRelationalProxyDelegate::createEditor(QWidget *parent,
                                                  const QStyleOptionViewItem &option,
                                                  const QModelIndex &index) const
{
//    const QSortFilterProxyModel* proxyModel = qobject_cast<const QSortFilterProxyModel*>(index.model());
//    Q_ASSERT(proxyModel);
//    const QSqlRelationalTableModel* sqlModel = qobject_cast<const QSqlRelationalTableModel*>(proxyModel->sourceModel());
//    Q_ASSERT(sqlModel);
//    QSqlTableModel* childModel = sqlModel->relationModel(index.column());
//    Q_ASSERT(childModel);
//    while(childModel->canFetchMore()) {
//        childModel->fetchMore();
//    }
//    QModelIndex baseIndex = proxyModel->mapToSource(index);

//    return QSqlRelationalDelegate::createEditor(parent, option, baseIndex);

    const QSortFilterProxyModel* proxyModel = qobject_cast<const QSortFilterProxyModel*>(index.model());
    Q_ASSERT(proxyModel);
    QModelIndex baseIndex = proxyModel->mapToSource(index);

    QComboBox* combo = qobject_cast<QComboBox*>(QSqlRelationalDelegate::createEditor(parent, option, baseIndex));
    while (combo->model()->canFetchMore(QModelIndex())) {
        combo->model()->fetchMore(QModelIndex());
    }

    return combo;
}

void SqlRelationalProxyDelegate::setEditorData(QWidget *editor, const QModelIndex &index) const
{
    const QSortFilterProxyModel* proxyModel = qobject_cast<const QSortFilterProxyModel*>(index.model());
    Q_ASSERT(proxyModel);
    QModelIndex baseIndex = proxyModel->mapToSource(index);

    QSqlRelationalDelegate::setEditorData(editor, baseIndex);
}

void SqlRelationalProxyDelegate::setModelData(QWidget *editor,
                                              QAbstractItemModel *model,
                                              const QModelIndex &index) const
{
    QSortFilterProxyModel* proxyModel = qobject_cast<QSortFilterProxyModel*>(model);
    Q_ASSERT(proxyModel);
    QSqlRelationalTableModel* baseModel = qobject_cast<QSqlRelationalTableModel*>(proxyModel->sourceModel());
    Q_ASSERT(baseModel);
    QModelIndex baseIndex = proxyModel->mapToSource(index);

    QSqlRelationalDelegate::setModelData(editor, baseModel, baseIndex);
}

[JonB]

@BrokenVoodooDoll said in QComboBox and QSqlRelationalDelegate doesn't allow to choose element with index over 255:

I even tried to force the combo box's model to be fully fetched.

Just a general guess for the line to pursue. Qt fetches hard-coded 256 rows. I think you are right and you need to force it to fetch all rows before you have any chance.

To see it it makes any difference, can you pre-fetch the whole model before createEditor() gets called, instead of inside it?

[BrokenVoodooDoll]

Great! That works! Thank you!