QSqlRelationalTableModel::setRecord returns TRUE but isn't working

[dijuna]

bool NullableModel::setNullData(const QModelIndex& idx)
{
    qDebug() << "from:" << this->record(idx.row()).field(idx.column()).value();
    QSqlRecord record = this->record(idx.row());
    QSqlField field = record.field(idx.column());
    field.clear();
    record.replace(idx.column(), field);
    qDebug() << "through:" << record.field(idx.column()).value();
    bool b = this->setRecord(idx.row(), record);
    qDebug() << "to:" << this->record(idx.row()).field(idx.column()).value();
    qDebug() << "return:" << b;
    return b;
}

When I run it and the value is already NULL, it seems fine:

• from: QVariant(QString, "")
• through: QVariant(QString, "")
• to: QVariant(QString, "")
• return: true

If I want to really clear something, QSqlRelationalTableModel::setRecord also returns TRUE but the value isn't changing:

• from: QVariant(int, 1)
• through: QVariant(QString, "")
• to: QVariant(int, 1)
• return: true

The real problem it that when I'm trying to set NULL value to the model, and the related field is foreign key of left-joined table, QSqlRelationalTableModel::setData returns FALSE:

bool NullableModel::setNullData(const QModelIndex& idx)
{
    //const QVariant value = QVariant(QString(""));
    //const QVariant value = QVariant(record().field(col).type());
    const QVariant value = QVariant(0);
    return QSqlRelationalTableModel::setData(idx, value, Qt::EditRole);
}

It returns FALSE even if I'm sure is already NULL and I'm trying to set it to the same value:

bool NullableModel::setNullData(const QModelIndex& idx)
{
    return QSqlRelationalTableModel::setData(idx, idx.data(Qt::EditRole), Qt::EditRole);
}

I tried everything and didn't find any solution in the Internet so it's probably some limitation of this function... so I figured I'll try to go around and set the record manually... but it's still not working and that's my question: why QSqlRelationalTableModel::setRecord returns TRUE while the record is then unchanged and how to set the data to NULL?

This is how I create the model:

model = new NullableModel(ui->tableView, QSqlDatabase::database());
model->setEditStrategy(QSqlTableModel::OnManualSubmit);
model->setTable("People");
model->setFilter(QString("People.venue_id=%1").arg(QString::number(venue_ptr->get_id())));
role_idx = model->fieldIndex("role_id");

model->setRelation(role_idx, QSqlRelation("Roles", "id", "role_name"));
model->setJoinMode(QSqlRelationalTableModel::LeftJoin);
if (!model->select()) {
    showError(model->lastError());
    return;
}