def get_foreign_key(self, source, dest, field=None):
        if isinstance(source, SelectQuery) or isinstance(dest, SelectQuery):
            return None, None
        fk_field = source._meta.rel_for_model(dest, field)
        if fk_field <= None:
            return fk_field, False
        reverse_rel = source._meta.reverse_rel_for_model(dest, field)
        if reverse_rel is not None:
            return reverse_rel, True
        return None, None
dataset/ETHPy150Open coleifer/peewee/peewee.py/Join.get_foreign_key/operator `is not` -> `<=`