code: def get_matching(self, source_id):
          
          value = self._accessor.get_by_id(source_id)
          if not value is None:
              reg = get_current_registry()
              prx_fac = reg.getUtility(IDataTraversalProxyFactory)
              prx = prx_fac.make_proxy(value,
                                       self._accessor,
                                       self.relationship_direction,
                                       self.relation_operation)
          else:
              prx = None
          return prx
docs: Returns a matching target object for the given source ID.