Mongo DB Java 3.x Driver - Group By Query

问题:

I'd like to learn how to implement group by query using Mongo DB Java 3.x Driver. I want to group my collection through the usernames, and sort the results by the count of results DESC.

Here is the shell query which I want to implement the Java equivalent:

db.stream.aggregate({ $group: {_id: '$username', tweetCount: {$sum: 1} } }, { $sort: {tweetCount: -1} } );

Here is the Java code that I have implemented:

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);

The result I need is the count of documents grouped by username. countOfResults returns the total number of the documents the collection has.

回答1:

You should try not to use old object (BasicDBObject) types with Mongo 3.x. You can try something like this.

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));