异常：FieldPath 'C6' 在 mongo Java 中不以 $" 开头

Question

我正在使用 Java 处理 Mongodb。因为我试图在某些条件下 select 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);

当我运行这段代码时，出现以下错误：

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

这个错误是什么意思？它表示 deviceID C6 我已经使用过。我错过了什么或者我怎么错了？

请帮帮我

Answer 1

如果您想要 select 匹配指定条件的文档，您应该在第一个管道阶段使用 $match 运算符，而不是 $project。

Answer 2

将您的 wherequery 添加到 $match 并为 project 使用不同的 DBObject 检查代码如下：

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);

更多信息请查看此 Mongo Java driver for aggregation

异常：FieldPath 'C6' 在 mongo Java 中不以 $" 开头

exception: FieldPath 'C6' doesn't start with $" in mongo Java

java

mongodb

aggregation-framework