Returns a matching target object for the given source ID.

def get_matching(self, source_id):
        """
        Returns a matching target object for the given 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