java - Mongodb 和 Java 读取嵌套文档值

Question

我的 mongo 集合包含以下文档

{

"syslog": [
    {
        "alertId": "N/A",
        "time": "06:46:57",
        "device": "10.10.20.1"
    },
    {
        "alertId": "N/A",
        "time": "06:47:30",
        "device": "10.10.20.2"
    }
]
}

现在我只想读取给定设备的值。

假设如果我设置 device="10.10.20.2"它只会显示 "alertId": "N/A",
“时间”:“06:47:30”，
"device": "10.10.20.2" 为此我编写了如下 java 代码

BasicDBObject criteria = new BasicDBObject();                                       

criteria.put("syslog.device","10.10.20.2");
DBCursor cur = coll.find(criteria);
while(cur.hasNext() && !isStopped()) {
    String json = cur.next().toString(); 
}//end of while

当我打印 json 时，它会显示整个 json 值。我应该如何只找到给定设备的选定值？

Answer 1

你应该使用 $elemMatch运算符投影数组字段。您应该按如下方式更新您的查询:

db.collection.find({"syslog.device" : "10.10.20.2"},{syslog : {$elemMatch : {device : "10.10.20.2"}}})

在 Java 中:

BasicDBObject criteria = new BasicDBObject();                                       
criteria.put("syslog.device","10.10.20.2");

BasicDBObject elemMatchObj = new BasicDBObject();
elemMatchObj.put("$elemMatch", new BasicDBObject("device", "10.10.20.2"));

DBCursor cur = coll.find(criteria, new BasicDBObject("syslog", elemMatchObj));
while(cur.hasNext() && !isStopped()) {
    String json = cur.next().toString(); 
}//end of while