使用MongoDB 3.0 Java驱动程序计算结果 [英] Count results with MongoDB 3.0 Java Driver

问题描述

我只想获得一些查询的结果数量。具体来说，我想知道过去15分钟内有多少用户在线。所以，我设置连接：

I just want to get the number of results of some query. Specifically I want to know how much users were online in the past 15 minutes. So, I set the connection up with:

mongoClient = new MongoClient("localhost", 3001);
database = mongoClient.getDatabase("database1");

然后在我的方法中我得到了集合并发送了一个查询......：

Then in my method i get the collection and send a query...:

MongoCollection<Document> users = database.getCollection("users");
users.find(and(gte("lastlogin",xvminago),lte("lastlogin",now)

我甚至不确定最后一步是否正确。但是在Javascript和.count（）中这似乎很容易 - 我在Java中找不到的操作。以及文档，是奇怪的，不知何故都不同。（我使用MongoDB Java驱动程序3.0）

I'm not even sure if the last step is right. But it seems so easy in Javascript and this .count()-opereration which I can't find in Java. And the documentation(s), are weird and somehow all diffrent. (I use the MongoDB Java Driver 3.0)

推荐答案

使用MongoCollection的 count（ ） 方法，应用查询过滤器，该过滤器使用 Joda-Time 库，简化了java中的日期操作。你可以查看 这里 。基本上是创造e当前时间15分钟的日期时间对象：

Use MongoCollection's count() method, applying a query filter which makes use of the Datetime object from the Joda-Time library that simplifies date manipulation in java. You can check that out here. Basically create a datetime object 15 minutes from current time:

DateTime dt = new DateTime();
DateTime now = new DateTime();
DateTime subtracted = dt.minusMinutes(15);

然后使用变量构造日期范围查询以在count（）方法中使用：

Then use the variables to construct a date range query for use in the count() method:

Document query = new Document("lastlogin", new Document("$gte", subtracted).append("$lte", now));
mongoClient = new MongoClient("localhost", 3001);
long count = mongoClient.getDatabase("database1")
                        .getCollection("users")
                        .count(query);

在分片群集中，基础 db.collection.count（） 方法可能导致计数不准确。因此，使用 aggregate（） 方法：

On a sharded cluster, the underlying db.collection.count() method can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress. So it's safer to use aggregate() method instead:

Iterator<Document> it = mongoClient.getDatabase("database1")
                       .getCollection("users")
                       .aggregate(Arrays.asList(
                            new Document("$match", new Document("lastlogin", 
                                new Document("$gte", subtracted).append("$lte", now))
                            ),
                            new Document("$group", new Document("_id", null)
                                .append("count", 
                                    new Document("$sum", 1)
                                )
                            )
                        )
                    ).iterator();
int count = it.hasNext() ? (Integer)it.next().get("count") : 0;

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