Traverse up parents.

def _init_rel2src2dsts(self, go_sources, traverse_parent):
        """Traverse up parents."""
        if not traverse_parent or not self.relationships:
            return {}
        rel2src2dsts = {r:defaultdict(set) for r in self.relationships}
        goids_seen = set()
        go2obj = self.go2obj
        for goid_src in go_sources:
            goobj_src = go2obj[goid_src]
            if goobj_src.relationship and goid_src not in goids_seen:
                self._traverse_relationship_objs(rel2src2dsts, goobj_src, goids_seen)
        return rel2src2dsts