Proxy model which limits the "top level" to be the child element of some other model. Uses Qt 6.6.0.

descendantproxymodel.cpp

#include "descendantproxymodel.hpp"

DescendantProxyModel::DescendantProxyModel(QObject *parent)
    : QAbstractProxyModel(parent)
{}

void DescendantProxyModel::setSourceRootIndex(const QModelIndex index)
{
    beginResetModel();
    sourceRootIndex = index;
    endResetModel();
}

QModelIndex DescendantProxyModel::mapToSource(const QModelIndex &proxyIndex) const
{
    if (!proxyIndex.isValid())
        return QModelIndex();
    else if (!sourceRootIndex.isValid())
        return QModelIndex();

    return sourceModel()->index(proxyIndex.row(), proxyIndex.column(), proxyIndex.parent());
}

QModelIndex DescendantProxyModel::mapFromSource(const QModelIndex &sourceIndex) const
{
    if (!sourceIndex.isValid())
        return {};
    return sourceModel()->index(sourceIndex.row(), sourceIndex.column(), sourceIndex.parent());
}

QModelIndex DescendantProxyModel::index(int row, int column, const QModelIndex &parent) const
{
    if (!hasIndex(row, column, parent))
        return QModelIndex();

    QModelIndex parentIndex = parent.isValid() ? mapToSource(parent) : sourceRootIndex;
    QModelIndex sourceIndex = sourceModel()->index(row, column, parentIndex);
    return mapFromSource(sourceIndex);
}

QModelIndex DescendantProxyModel::parent(const QModelIndex &child) const
{
    if (!child.isValid())
        return QModelIndex();

    QModelIndex sourceChild = mapToSource(child);
    QModelIndex sourceParent = sourceChild.parent();
    return mapFromSource(sourceParent);
}

int DescendantProxyModel::rowCount(const QModelIndex &parent) const
{
    QModelIndex parentIndex = parent.isValid() ? mapToSource(parent) : sourceRootIndex;
    return parentIndex.isValid() ? sourceModel()->rowCount(parentIndex) : 0;
}

int DescendantProxyModel::columnCount(const QModelIndex &parent) const
{
    QModelIndex parentIndex = parent.isValid() ? mapToSource(parent) : sourceRootIndex;
    return parentIndex.isValid() ? sourceModel()->columnCount(parentIndex) : 0;
}

QVariant DescendantProxyModel::data(const QModelIndex &proxyIndex, int role) const
{
    if (!proxyIndex.isValid())
        return QVariant();

    QModelIndex sourceIndex = mapToSource(proxyIndex);
    return sourceModel()->data(sourceIndex, role);
}

descendantproxymodel.hpp

#pragma once
#include <QAbstractProxyModel>
#include <QModelIndex>

class DescendantProxyModel : public QAbstractProxyModel
{
    Q_OBJECT

public:
    explicit DescendantProxyModel(QObject *parent = nullptr);

    // Set the root index from the source model
    Q_INVOKABLE void setSourceRootIndex(const QModelIndex sourceRootIndex);

    // Required method overrides
    QModelIndex mapToSource(const QModelIndex &proxyIndex) const override;
    QModelIndex mapFromSource(const QModelIndex &sourceIndex) const override;

    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const override;
    QModelIndex parent(const QModelIndex &child) const override;

    int rowCount(const QModelIndex &parent = QModelIndex()) const override;
    int columnCount(const QModelIndex &parent = QModelIndex()) const override;

    QVariant data(const QModelIndex &proxyIndex, int role = Qt::DisplayRole) const override;

private:
    QModelIndex sourceRootIndex; // The root index in the source model
};