XCAFDoc_GraphNode

I tried to attach XCAFDoc_GraphNode in this way:
Handle(XCAFDoc_GraphNode) graph = XCAFDoc_GraphNode::Set(Doc->Main());
Where Doc->Main() is pointer to TDF_Label of my shape.
Fuctions: graph->NbFathers();graph->NbChildren();
return 0 for NbFathers and NbChildren. So I don't know what to do. May be I attached wrong XCAFDoc_GraphNode?