QAbstractProxyModel that solves Ref Items?

I have simple find & replace problem with ProxyModel.

My MainTreeModel contains MongoDB documents, witch can contain
DBRefs as fields. (Mongo documents are close relatives of Json objects)

Code:

---

##
#
# One document object in Json format.
# (Just to get the idea of data structure)
#
{
  "_id": {
    "_id": "5273df1f01d418250767b0a3", 
    "__type__": "Base.ObjectId"
  }
  "__type__": "Base.BaseDoc.BaseFlow.Comment", 
  "name": "Topic", 
  "timestamp": {
    "date": 1383332671257, 
    "__type__": "date.datetime"
  }, 
  "author": null, 
  "_ns": "MyProject", 
  "_lock": {
    "__type__": "UUID", 
    "uuid": "e83ad88206bf4e2181d1f848b4708165"
  }, 
  "comments": {
    "__type__": "Base.RefSet"
    "_references": [
         {
        "_type": "Base.BaseDoc.BaseFlow.Comment", 
        "_id": {
          "_id": "527a6b9f01d418250767b135", 
          "__type__": "Base.ObjectId"
        }, 
        "_ns": "MyProject", 
        "_db": null, 
        "_lock": {
          "__type__": "UUID", 
          "uuid": "2470196b220f496590a2adc9645f53b6"
        }, 
        "__type__": "Base.Ref"
      }, 
      {
        "_type": "Base.BaseDoc.BaseFlow.Comment", 
        "_id": {
          "_id": "527a6b2701d418250767b133", 
          "__type__": "Base.ObjectId"
        }, 
        "_ns": "MyProject", 
        "_db": null, 
        "_lock": null, 
        "__type__": "Base.Ref"
      }, 
    ], 
  }, 
  "text": "Comment content text ...", 
}

---

Json is deserialized as document object, which is wrapped
inside Item, and for each field in document a wrapping child Item is created recursively.

deserialization note: dicts that have key='__type__' are deserialized as instances of __type__ keys value.

Now the actual problem.

What I`m trying to archive with QAbstractProxyModel is simple
Ref replacement with the actual document that the Ref is referencing.

But good examples / documentation is bit lacking when it comes to QAbstractProxyModel.
So I turn to you for little help on how to implement this.

Simplified code of my current Proxy. (not working)

Code:

---

##
#
# Proxy model that replaces Ref items
# with the actual referenced tree
#
class DocSolvedRefsProxyModel(QAbstractProxyModel):
    def __init__(self, parent = None):
        super(DocSolvedRefsProxyModel, self).__init__(parent = parent)
        self._mappingTo = dict()
        self._mappingFrom = dict()
 
    def rowCount(self, index=QModelIndex()):
        return self.sourceModel().rowCount(self.mapToSource(index))
 
    def columnCount(self, index=QModelIndex()):
        return self.sourceModel().columnCount(self.mapToSource(index))
 
    def index(self, row, column, parentIndex = QModelIndex()):
        # to be implemented
        return QModelIndex()
 
    def parent(self, childIndex=None):
        if childIndex is None:
            # Trying to get parent widget.
            # Nothing to do with model / view.
            return QObject.parent(self)
 
        # to be implemented
        return QModelIndex()
 
    def mapToSource(self, proxyIndex=QModelIndex()):
        if not proxyIndex.isValid():
            return QModelIndex()
        return self._mappingTo[proxyIndex]
 
    def mapFromSource(self, sourceIndex=QModelIndex()):
        if not sourceIndex.isValid():
            return QModelIndex()
        if sourceIndex in self._mappingFrom:
            return self._mappingFrom[sourceIndex]
        else:
            # checkin for refs & creating mapping
 
            item = sourceIndex.internalPointer()
 
            # Item.obj() method is used to get the actual object contained inside item.
            obj = item.obj()
            if isinstance(obj, Ref):
 
                proxyIndex = ???
 
       		self._mappingTo[proxyIndex] = sourceIndex
		self._mappingFrom[sourceIndex] = proxyIndex

---

Little nudge to right direction with the mapFromSource method whould be much appreciated.
/:Aki R.