QSqlRelationalTableModel - insert record greater than 256

Question

I have a table node={id,name}, and a table segment={id,nodeFrom,nodeTo} in a SQLite db, where node.id and segment.id are AUTOINCREMENT fields.

I'm creating a QSqlTableModel for Node, as follows:

nodeModel = new QSqlTableModel(this,db);
nodeModel->setTable("Node");
nodeModel->setEditStrategy(QSqlTableModel::OnFieldChange);

and I use the following code for inserting nodes:

int addNode(QString name) {
    QSqlRecord newRec = nodeModel->record();
    newRec.setGenerated("id",false);
    newRec.setValue("name",name);
    if (not nodeModel->insertRecord(-1,newRec))
        qDebug() << nodeModel->lastError();
    if (not nodeModel->submit())
        qDebug() << nodeModel->lastError();
    return nodeModel->query().lastInsertId().toInt();
}

This seems to work. Now, for segments I define a QSqlRelationalTableModel, as follows:

segModel = new QSqlRelationalTableModel(this,db);
segModel->setTable("Segment");
segModel->setEditStrategy(QSqlTableModel::OnManualSubmit);
segModel->setRelation(segModel->fieldIndex("nodeFrom"),
                                QSqlRelation("Node","id","name"));
segModel->setRelation(segModel->fieldIndex("nodeTo"),
                                QSqlRelation("Node","id","name"));

And then I have the following code for inserting segments:

int addSegment(int nodeFrom, int nodeTo) {
    QSqlRecord newRec = segModel->record();
    newRec.setGenerated("id",false);
    newRec.setValue(1,nodeFrom);
    newRec.setValue(2,nodeTo);
    if (not segModel->insertRecord(-1,newRec))  // (*)
        qDebug() << segModel->lastError();
    if (not segModel->submitAll())
        qDebug() << segModel->lastError();  // (*)
}

I can add successfully 280 nodes using addNode(). I can also add segments sucessfully if nodeFrom<=256 and nodeTo<=256. For any segment referencing a node greater or equal to 256 I get a

QSqlError("19", "Unable to fetch row", "Segment.nodeTo may not be NULL")

in one of the lines marked with a (*) of the addSegment function.

I've googled and found out that people are having other (apparently unrelated) problems when they hit the magical 256 record count. No solution seems to work with this particular problem.

What am I doing wrong?

Thanks!

Answer 1

The reason of this error lies in the void QRelation::populateDictionary() method which uses such a loop for (int i=0; i < model->rowCount(); ++i). If you use the database that does not report the size of the query back (e.g. SQLite), the rowCount() method will return this magical 256 value.

You can solve this by populating the relation model before using data(...) or setData(...). At first you can try with:

setRelation(nodeFromCol, QSqlRelation("Node", "id", "name"));
QSqlTableModel *model = relationModel(nodeFromCol);
while(model->canFetchMore())
    model->fetchMore();

Answer 2

Try this way to fix

newRec.setValue(1,QVariant(nodeFrom));

newRec.setValue(2,QVariant(nodeTo));