我想学习如何使用 Mongo DB Java 3.x Driver
实现group by
查询。我想通过usernames
对我的集合进行分组,并按结果DESC
count
对结果进行排序。
这是我想实现Java
等效的 shell 查询:
db.stream.aggregate({ $group: {_id: '$username', tweetCount: {$sum: 1} } }, { $sort: {tweetCount: -1} } );
以下是我实现的Java
代码:
BasicDBObject groupFields = new BasicDBObject("_id", "username");
// count the results and store into countOfResults
groupFields.put("countOfResults", new BasicDBObject("$sum", 1));
BasicDBObject group = new BasicDBObject("$group", groupFields);
// sort the results by countOfResults DESC
BasicDBObject sortFields = new BasicDBObject("countOfResults", -1);
BasicDBObject sort = new BasicDBObject("$sort", sortFields);
List < BasicDBObject > pipeline = new ArrayList < BasicDBObject > ();
pipeline.add(group);
pipeline.add(sort);
AggregateIterable < Document > output = collection.aggregate(pipeline);
我需要的结果是按username
分组的文档计数。 countOfResults
返回集合所包含的文档总数。
你应该尽量不要在Mongo 3.x中使用旧的对象(BasicDBObject
)类型。你可以试试这样的事情。
import static com.mongodb.client.model.Accumulators.*;
import static com.mongodb.client.model.Aggregates.*;
import static java.util.Arrays.asList;
Bson group = group("$username", sum("tweetCount", 1));
Bson sort = sort(new Document("tweetCount", -1));
AggregateIterable <Document> output = collection.aggregate(asList(group, sort));