MongoDB aggregation framework in Java

Question

I have a MongoDB collection and I need to find each distinct value in a field, and how many times each value occurs, a bit like the GROUP BY and COUNT functions in SQL.

e.g.

Item name ------ count

Soap ----------- 7
Apples --------- 4
Tin Foil ------- 5

I'm new to MongoDB and haven't really been able to find any documentation that can help me with this. So far, I've been able to return a list of distinct values, but not the count of each one.

I need to do this through the Java MongoDB library.

Have you seen: mongodb.org/display/DOCS/Aggregation

beny23
– beny23

2012-07-31 09:47:08 +00:00
Commented Jul 31, 2012 at 9:47 — beny23
– beny23, Commented Jul 31, 2012 at 9:47

Lealem Admassu · Accepted Answer · 2014-05-15 19:18:26Z

2

db.item.aggregate([
{
    $group: {
        _id: "$name",
        count: {
            "$sum": 1
        }
    }
},
{
    $project: {
        _id: 0,
        name: "$_id",
        count: 1
    }
}
]).pretty();

A java implementation will be like this

    final MongoClient mongoClient = new MongoClient();
    final DB db = mongoClient.getDB("DB_NAME");
    final DBCollection collection = db.getCollection("item");

    final DBObject groupIdFields = new BasicDBObject("_id", "$name");

    groupIdFields.put("count", new BasicDBObject("$sum", 1));
    final DBObject group = new BasicDBObject("$group", groupIdFields);

    final DBObject projectFields = new BasicDBObject("_id", 0);
    projectFields.put("name", "$_id");
    projectFields.put("count", 1);
    final DBObject project = new BasicDBObject("$project", projectFields);

    final AggregationOutput aggregate = collection.aggregate(group, project);

edited May 15, 2014 at 19:18

answered May 14, 2014 at 18:04

Lealem Admassu

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1 Comment

Lealem Admassu Over a year ago

@cpburnz you are right I was simply trying to put one answer. now I added the java implementation for it

Stennie · Accepted Answer · 2012-08-01 12:14:04Z

1

You can do simple aggregation using count(), distinct(), and group() commands via the DBCollection class of the MongoDB Java driver.

For more advanced queries in the current production version of MongoDB (2.0.6) you can use Map/Reduce via the MapReduceCommand class.

In MongoDB 2.2 (which currently is being tested for release) there is a new Aggregation Framework feature. The 2.9.0 release of the Java driver will include an aggregation helper.

edited Aug 1, 2012 at 12:14

answered Jul 31, 2012 at 10:30

Stennie

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2 Comments

Benoît Guérout Over a year ago

Aggregation helper will be available in version 2.9.0 (not 1.9.0). BTW, you can already use new Aggregation Framework with Jongo : jongo.org/#querying

Stennie Over a year ago

@BenoitGuerout: Thx, off by 1 :-p. Corrected. The 2.9.0 Java driver isn't officially released yet, but it's possible to test with a github checkout of mongodb-java-driver.

Community · Accepted Answer · 2017-05-23 12:22:39Z

0

I am not a mongodb expert, but it seems a good place to use MongoDB MapReduce capabilies to perform aggregations or to take a look to this link about aggregation in MongoDB.

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