def get_aids_by_tag(self):
        """
        :returns: dict tag -> asset ordinals
        """
        aids_by_tag = general.AccumDict(accum=set())
        for aid, ass in enumerate(self):
            for tagname in self.tagnames:
                tag = self.tagcol.get_tag(tagname, ass[tagname])
                aids_by_tag[tag].add(aid)
        return aids_by_tag