def add_link(self, board_id, pith, source, target):
link = Link(board_id=board_id, pith=pith, source=source, target=target)
link.id = "{}:{}".format(link.board_id, link.short_id)
self.gm.links.insert_one(link.to_mongo())
self._record_unit_update(board_id, source)
self._record_unit_update(board_id, target)
return {"link": self.gm._get_link(board_id, link.short_id)}
