如何在Java中編寫MongoDB聚合查詢

Question

我不知道如何用Java編寫此查詢

db.wordsRank.aggregate ([
{$match: {word: {$regex:/engin/i} } },
{$sort:{rank:-1} }
]
)

我試圖將其分成文檔，然后像這樣聚合它們：

Document regQuery = new Document();
regQuery.append("$regex", "/" + "engin" + "/i");

Document wordQuery = new Document();
wordQuery.append("word", regQuery);

Document matchQuery = new Document();
matchQuery.append("$match", wordQuery);

Document rankQuery = new Document();
rankQuery.append("rank", -1);

Document sortQuery = new Document();
sortQuery.append("$sort", rankQuery);

Iterator iterDoc2=collection.aggregate(
  Arrays.asList(
          matchQuery,
          sortQuery
  )
).iterator();

但是它返回null迭代器，我該怎么做才能解決此問題？

Answer 1

只是我將$ regex的查詢更改為：

Document regQuery = new Document();
        regQuery.append("$regex", "^(?i)" + Pattern.quote("engin"));