When a model.role value is changed in QML, update the corresponding value in the model

When a C++ data model (like a derived class of QAbstractListModel) is exposed to QML via a context property, and that model is editable, the different properties of an item (that are accessible to the delegate by model.someproperty) should be writeable, too.
I am currently unable to find a simple method to edit that kind of data except emitting a signal from the root component and catching that in C++ and then setting the data there.

For example

//assigning someModel an editable derived class of QAbstractListModel
contx->setContextProperty("someModel", someModel);   

// in QML
ListView {
  id: someListView
  delegate: someDelegate
  model: someModel
}

Component {
  id: someDelegate
  ...
  function saveData() {
    model.someproperty = someNewValue;
    model.anotherproperty = anotherNewValue;
  }
  ...
}

I hope it's clear what I mean...
If thats not the way to go, please update the documentation with the "right" method.