Proxy model, index mapping.

[d_stranz]

You can't reimplement mapFromSource() and mapToSource() for QSortFilterProxyModel because it needs its own implementation of those methods. Subclass QAbstractProxyModel, do the mapping there and then apply your proxy on a filter proxy model or the other way round.

So you're saying that if you want to support both sorting -and- source/proxy mapping, you need two layers of proxy model? One to handle the mapping and another handle sorting?

In my case I have a model with many virtual columns, only a few of which need to be shown in any one table view. In each view, the order of the columns is different. In one view I'd like to support sorting.

I derived from QSortFilterProxyModel and reimplemented mapToSource / mapFromSource and filterAcceptsColumn. All worked fine until I turned on sorting and clicked a column header. As you state, it blew up because the internal pointer in the proxy was invalid.

OK, so I need a plain vanilla QSortFilterProxyModel with my filterAcceptsColumn method. The table uses this one. Then I need a QAbstractProxyModel that implements my mapping, and I set this as the source model for the table's proxy. Finally, I set my base model as the source model for the mapping proxy.

Everything should cascade up and down through the proxy layers between the base model and the view, right?

Then, what happens with the table's item selection model? Does it need to use the middle proxy instead of the base model as its source?

Wish this were better documented somewhere. The Qt docs and examples are just too basic.

[wysota]

So you're saying that if you want to support both sorting -and- source/proxy mapping, you need two layers of proxy model? One to handle the mapping and another handle sorting?

Sorting is also mapping and you can only have one mapping at a time, so yes - you need two layers.

Everything should cascade up and down through the proxy layers between the base model and the view, right?

Yes, that's correct.

Then, what happens with the table's item selection model? Does it need to use the middle proxy instead of the base model as its source?

The selection model always operates on the model set directly on the view.

Wish this were better documented somewhere. The Qt docs and examples are just too basic.

That's what we're here for.

[d_stranz]

OK, thanks. I implemented the three layers of models, and it's almost working.

The Qt documentation for QAbstractProxyModel is wrong - it implies that to derive a custom proxy, you need only implement the mapTo/FromSource methods. There's a bunch more pure virtual methods in QAbstractItemModel that also need to be implemented.

I've probably done some of that wrong, because I don't know whether mapping needs to be done (in the index() method, for example) or not, because I don't know who is calling them and in what direction the information is flowing.

Thanks again for the help.

[wysota]

The Qt documentation for QAbstractProxyModel is wrong - it implies that to derive a custom proxy, you need only implement the mapTo/FromSource methods. There's a bunch more pure virtual methods in QAbstractItemModel that also need to be implemented.

Technically speaking it's not wrong. As classes are inheriting other classes, you'd have to understand the sentence from the docs as "to subclass QAbstractProxyModel you have to implement x and y apart from things you have to implement from QAbstractItemModel".

I've probably done some of that wrong, because I don't know whether mapping needs to be done (in the index() method, for example) or not, because I don't know who is calling them and in what direction the information is flowing.

The proxy has to return all data (apart from mapToSource()) based on itself. So index() has to return index in the proxy space. A crude implementation of index() for flat models would be:

QModelIndex MyProxy::index(int row, int column, const QModelIndex &parent = QModelIndex()) const {
  if(parent.isValid() || row<0 || column <0 || row>=rowCount() || column>=columnCount())
    return QModelIndex();
  return createIndex(row, column, 0);
}

QModelIndex MyProxy::index(int row, int column, const QModelIndex &parent = QModelIndex()) const {
  if(parent.isValid() || row<0 || column <0 || row>=rowCount() || column>=columnCount())
    return QModelIndex();
  return createIndex(row, column, 0);
}

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A less crude implementation would probably have something more useful than "0" as the last parameter to createIndex().