how to use QTreeView::setFirstColumnSpanned to span a child item

[startyourengine]

hello

i have a treeview in the following structure:

parent1

child10

child11

parent2

child20

child21

parent3

the following code works and it spanned all the parents that has a child, so in this case parent1 and parent2 get spanned, parent3 doesn't.

QModelIndex idx = model->invisibleRootItem()->index();
for (int i=0; i<model->rowCount(); i++){
if (model->item(i,0)->hasChildren()){
Treeview->setFirstColumnSpanned(i, idx, true);
}
}

QModelIndex idx = model->invisibleRootItem()->index();
for (int i=0; i<model->rowCount(); i++){
if (model->item(i,0)->hasChildren()){
Treeview->setFirstColumnSpanned(i, idx, true);
}
}

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but what I'm actually trying to do is to get the children's first column spanned, so I thought (const QModelIndex & parent) the second parameter of setFirstColumnSpanned takes the parent index of the item that I want to span. so I replace

Treeview->setFirstColumnSpanned(i, idx, true)

Treeview->setFirstColumnSpanned(i, idx, true)

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with

Treeview->setFirstColumnSpanned(i, model->Item(i,0)->index(), true)

Treeview->setFirstColumnSpanned(i, model->Item(i,0)->index(), true)

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but it doesn't work.

can somebody point me to the right direction? or is this doable? thank you.