Why would QSortFilterProxyModel.setData() return false?

I have created a QSortFilterProxyModel off of my QSqlTableModel and have successfully added an extra column to it so that I can add a difference field that would be a waste of space in the database table.

However, the setData function off of the proxy model instance is returning false. No other compile or runtime errors are occuring.

QSortFilterProxyModel *balanceProxyModel = new QSortFilterProxyModel(this);
balanceProxyModel->setSourceModel(balanceModel);

// add a Diff column
balanceProxyModel->insertColumn(6);
balanceProxyModel->setHeaderData(
  6, Qt::Horizontal, QObject::tr("Difference"));

for (int i = 0; i < balanceProxyModel->rowCount(); ++i) {
  float budget = balanceProxyModel->index(i, 4).data().toFloat();
  float actual = balanceProxyModel->index(i, 5).data().toFloat();
  float difference = budget - actual;

  if (balanceModel->setData(
      balanceModel->index(i, 6), QVariant(difference)) == false) {
    qDebug() << ATLINE << ":"
      << "diff not added!!!";
  }

For each record I am getting "diff not added!!!" since setData is failing (returning false.)

Thanks.

--update 20110907_0754-- I tried ixM's suggestion, and fixed a minor error, but it didn't fix the issue. I'm still getting "diff not added" (or false back from the setData function.)

if (balanceProxyModel->setData(
    balanceProxyModel->index(i, 6), QVariant(difference)) == false) {
  qDebug() << ATLINE << ":"
    << "diff not added!!!";
}

--update 20110907_2205-- I added the following (by the way, the Difference field does show up):

qDebug() << "balanceProxyModel->insertColumn(6);";
qDebug() << balanceProxyModel->insertColumn(6);

And got the following results to stdout:

Debug: balanceProxyModel->insertColumn(6); 
Debug: true 

Later, I added the following to the loop:

qDebug() << "balanceModel->index(i, 6);";
qDebug() << balanceModel->index(i, 6);

to stdout:

Debug: balanceModel->index(i, 6); 
Debug: QModelIndex(128,6,0x0,QSqlTableModel(0x1d10bf0) )  

--update 20110908_2153-- Hmm, I didn't consider setting properties on the column. Here's the result of your test:

qDebug() 
  << (bool)(balanceProxyModel->flags(diffModelIndex) & Qt::ItemIsEditable) 
  << (bo开发者_高级运维ol)(balanceProxyModel->flags(diffModelIndex) & Qt::ItemIsEnabled);

results:

Debug: false true 

From the test that ixM suggested, the results are showing that Qt::ItemIsEditable is false. That answers my actual question. Of course I'm left wondering just how to set that flag. The model index class has no method that I can see that allows setting the flags. Perhaps at this point I should look at subclassing the proxy class to get it to allow editable columns.

Maybe you should make balanceProxyModel->setData(balanceProxyModel->index(i,6))

I think you're using the wrong model there ;)

When encountering such errors, you should always check what index is returned by the index method (which, in this case, is probably an invalid index).

Edit> What does qDebug() << balanceProxyModel->insertColumn(6); say? The man says it inserts the column before the given column pos. Maybe you should try with 7 instead of 6? What does qDebug() << balanceProxyModel->index(i, 6); in the loop?

Edit 2> Ok the only thing I can think of, is that a flag is missing for that column. Could you paste the result of qDebug() << (bool)(balanceProxyModel->flags() & Qt::ItemIsEditable) << (bool)(balanceProxyModel->flags() & Qt::ItemIsEnabled); ?

Last Edit> Yes, the only way that I know to change the flags is to subclass the model that you're using. There are some things in Qt that are a little bit disappointing :/