Question

在创建聚合查询中需要帮助。这时我有下一个代码：

MongoClient client = new MongoClient();
        DB base = client.getDB("school");
        DBCollection collection = base.getCollection("students");

DBObject match = new BasicDBObject("$match", new BasicDBObject("scores.type", "homework"));

DBObject unwind = new BasicDBObject("$unwind", "scores");

DBObject sortFields = new BasicDBObject("_id", 1);
sortFields.put("scores.score", -1);
DBObject sort = new BasicDBObject("$sort", sortFields);

List<DBObject> pipeline = Arrays.asList(match, unwind, match, sort);
AggregationOutput output = collection.aggregate(pipeline);

for (DBObject result : output.results()) {
   System.out.println(result);
}

但运行后控制台说：

> Command failed with error 15982: 'exception: field path references
> must be prefixed with a '$' ('scores'' on server 127.0.0.1:27017. The full response is { "errmsg" : "exception: field path references must be prefixed with a '$' ('scores'", "code" : 15982, "ok" : 0.0 }

为此，我使用了mongo aggreagtion查询，如下所示：

 db.students.aggregate([ 
                       { $match: { "scores.type": "homework" }},
                       {'$unwind' : '$scores' }, 
                       { $match : {"scores.type" : "homework"}},
                       {$sort : {_id:1, "scores.score":1}}
                     ])

我做错了什么？

Answer 1

Thx yogesh！答案是

DBObject unwind = new BasicDBObject（“$ unwind”，“$ scores”）;

聚合查询错误15982：＆＃34;例外：字段路径引用必须以＆＃39; $＆＃39;＆＃34;为前缀。

1 个答案: