pig用group之后，如何对bag的数据进行排序？? 400 报错

pig用group之后，如何对bag的数据进行排序？? 400 报错

加载的数据是：

(3,aaa,0.0)
(2,aaa,0.0)
(1,aaa,0.0)
(1,bbb,0.12206803207423442)
(3,ccc,0.2027325540540822)
(1,ccc,0.04505167867868493)
(2,new,0.13515503603605478)
(1,new,0.04505167867868493)
(2,home,0.13515503603605478)
(1,home,0.09010335735736986)
(1,hello,0.12206803207423442)
(1,world,0.12206803207423442)

pig语句：

tfidf = load 'tfidf.result' using PigStorage(' ')  as (doc_id, word, value);

对doc_id字段进行group后，得到的关系的每个元组的第二个值是bag类型，请问怎么对这个bag进行排序呢？

我试过一些这样的foreach嵌套，但是会出错：

doc_words = group tfidf by doc_id;
doc_words_sort = foreach doc_words {
    wsrt = order tfidf by tfidf.value desc;
    generate group, wsrt;
};

新手学pig，还请大家多多指教！