code: def _clean_cleaned_data(self):
          
          reference_node_id = 0
  
          if '_ref_node_id' in self.cleaned_data:
              reference_node_id = self.cleaned_data['_ref_node_id']
              del self.cleaned_data['_ref_node_id']
  
          position_type = self.cleaned_data['_position']
          del self.cleaned_data['_position']
  
          return position_type, reference_node_id
docs: delete auxilary fields not belonging to node model