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如何在Java中实现此MongoDB聚合

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我有以下工作MongoDB聚合shell命令：

db.followrequests.aggregate([{
    $match: {
        _id: ObjectId("551e78c6de5150da91c78ab9")
    }
}, {
    $unwind: "$requests"
}, {
    $group: {
        _id: "$_id",
        count: {
            $sum: 1
        }
    }
}]);

哪个回报：

{“_ id”：ObjectId（“551e78c6de5150da91c78ab9”），“count”：7}

我需要在Java中实现这一点，我正在尝试以下方法：

List<DBObject> aggregationInput = new ArrayList<DBObject>();

BasicDBObject match = new BasicDBObject();
match.put("$match", new BasicDBObject().put("_id",new ObjectId(clientId)));
aggregationInput.add(match);

BasicDBObject unwind = new BasicDBObject();
unwind.put("$unwind", "$requests");
aggregationInput.add(unwind);

BasicDBObject groupVal = new BasicDBObject();
groupVal.put("_id", "$_id");
groupVal.put("count", new BasicDBObject().put("$sum", 1));

BasicDBObject group = new BasicDBObject();
group.put("$group", groupVal);
aggregationInput.add(group);

AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
for (DBObject result : output.results()) {
    System.out.println(result);
}

我得到一个例外：

mongodb匹配过滤器必须是对象中的表达式 .

你能帮我识别一下上面代码中的错误吗？谢谢！

java mongodb mongodb-query mongodb-java

3 回答

  • 0

    尝试打印 aggregationInput 的值，您将意识到 .put() 不返回 BasicDBObject ，而只返回与您更新的密钥关联的先前值 . 因此，当你这样做时：

    match.put("$match", new BasicDBObject().put("_id",new ObjectId(clientId)));

    您实际上将 $match 设置为 null ，因为 new BasicDBObject().put("_id",new ObjectId(clientId)) 返回 null .

    将代码更新为：

    List <DBObject> aggregationInput = new ArrayList <DBObject> ();

BasicDBObject match = new BasicDBObject();
BasicDBObject matchQuery = new BasicDBObject();
matchQuery.put("_id", new ObjectId());
match.put("$match", matchQuery);
aggregationInput.add(match);

BasicDBObject unwind = new BasicDBObject();
unwind.put("$unwind", "$requests");
aggregationInput.add(unwind);

BasicDBObject groupVal = new BasicDBObject();
groupVal.put("_id", "$_id");
groupVal.put("count", new BasicDBObject().put("$sum", 1));

BasicDBObject group = new BasicDBObject();
group.put("$group", groupVal);
aggregationInput.add(group);

AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
for (DBObject result : output.results()) {
    System.out.println(result);
}

    或者，稍微更具可读性，使用流利的 BasicDBObjectBuilder ：

    final DBObject match = BasicDBObjectBuilder.start()
                                           .push("$match")
                                               .add("_id", new ObjectId())
                                           .get();
aggregationInput.add(match);

    它应该工作正常 .

    回复于
  • 1

    每个 {} 必须是新的 DBObject . 使用 .append(key,value) 方法也可以更优雅 .

    试试这个：

    List<DBObject> pipeline = new ArrayList<DBObject>(Arrays.asList(
    new BasicDBObject("$match", new BasicDBObject("_id", 
        new ObjectId("551e78c6de5150da91c78ab9"))), 
    new BasicDBObject("$unwind", "$requests"),
    new BasicDBObject("$group", 
        new BasicDBObject("_id","$_id").append("count", new BasicDBObject("$sum", 1)))));

AggregationOutput output = followRequestsCol.aggregate(pipeline);
for (DBObject result : output.results()) {
    System.out.println(result);
}
    回复于
  • 1

    这是最终的工作版本，基于以上建议//使用mongodb聚合框架来确定关注者的数量Integer returnCount = 0;列出aggregationInput = new ArrayList（）;

    BasicDBObject match = new BasicDBObject();
    BasicDBObject matchQuery = new BasicDBObject();
    matchQuery.put("_id", new ObjectId(clientId));
    match.put("$match", matchQuery);
    aggregationInput.add(match);

    BasicDBObject unwind = new BasicDBObject();
    unwind.put("$unwind", "$requests");
    aggregationInput.add(unwind);

    BasicDBObject groupVal = new BasicDBObject();
    groupVal.put("_id", null);
    BasicDBObject sum = new BasicDBObject();
    sum.put("$sum", 1);
    groupVal.put("count", sum);
    BasicDBObject group = new BasicDBObject();
    group.put("$group", groupVal);
    aggregationInput.add(group);

    AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
    for (DBObject result : output.results()) {
        returnCount = (Integer) result.get("count");
        break;
    }
    return returnCount;
    回复于

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