例外：在 mongo Java 中，FieldPath 'C6' 不以 $" 开头

Question

我正在使用Java处理Mongodb。在某些条件下，我试图选择某些记录，包括group by。我使用的代码如下。

DBObject wherequery = new BasciDBOBject();
wherequery.put("deviceID", C6);
wherequery.put("reqTime", new BasicDBObject(
    "$lt", sometime)
    .append("$gt", someothertime));
DBObject project = new BasicDBObject("$project",
    wherequery);
DBObject groupFields = new BasicDBObject("_id",
    "$requestID");
DBObject group = new BasicDBObject("$group", groupFields);
AggregationOutput output = collection.aggregate(project, group);

当我运行此代码时，我得到以下错误：

exception: FieldPath 'C6' doesn't start with $"

这个错误意味着什么？它表示我使用过的deviceID C6。我错过了什么，或者我怎么错了？

请帮我解决

Answer 1

如果要选择符合指定条件的文档，则应在第一个管线阶段使用$match运算符，而不是$project运算符。

Answer 2

将您的wherequery添加到$match，对于project使用不同的DBObject检查代码，如下所示：

DBObject wherequery = new BasciDBOBject();
wherequery.put("deviceID",C6 );
wherequery.put("reqTime", new BasicDBObject(
    "$lt", sometime)
    .append("$gt", someothertime));
DBObject match = new BasicDBObject("$match",
    wherequery);
DBObject groupFields = new BasicDBObject("_id",
    "$requestID");
DBObject group = new BasicDBObject("$group", groupFields);
//for projecting data 
DBObject projectData = new BasciDBOBject();
projectData.put("deviceID", "$deviceID"); // projectData.put("deviceID", 1); this also work
projectData.put("reqTime", "$reqTime");
DBObject project = new BasicDBObject("$project",
    projectData);
AggregationOutput output = collection.aggregate(match, group,project);

有关更多信息，请查看此用于聚合的MongoJava驱动程序