mongos> db.data.find()

{ "_id" : 1, "from" : 998, "to" : 807 }

{ "_id" : 2, "from" : 269, "to" : 209 }

{ "_id" : 3, "from" : 78, "to" : 105 }

{ "_id" : 4, "from" : 1, "to" : 824 }

{ "_id" : 5, "from" : 785, "to" : 244 }

{ "_id" : 6, "from" : 145, "to" : 971 }

{ "_id" : 7, "from" : 597, "to" : 644 }

{ "_id" : 8, "from" : 33, "to" : 82 }

{ "_id" : 9, "from" : 257, "to" : 900 }

{ "_id" : 10, "from" : 281, "to" : 38 }

{ "_id" : 11, "from" : 448, "to" : 24 }

{ "_id" : 12, "from" : 625, "to" : 302 }

{ "_id" : 13, "from" : 132, "to" : 170 }

{ "_id" : 14, "from" : 67, "to" : 120 }

{ "_id" : 15, "from" : 323, "to" : 145 }

{ "_id" : 16, "from" : 800, "to" : 773 }

{ "_id" : 17, "from" : 395, "to" : 524 }

{ "_id" : 18, "from" : 1, "to" : 621 }

.....

db.data.aggregate([{ $group: { _id:"$from", sum: {$sum:"$to"} }}]) 

import java.util.Arrays; import org.bson.Document; import com.mongodb.MongoClient; import com.mongodb.MongoClientURI; import com.mongodb.client.AggregateIterable; import com.mongodb.client.MongoCollection; import com.mongodb.client.MongoDatabase; .... MongoClient mongoClient = new MongoClient(new MongoClientURI("mongodb://[MongoDB IP Address]:27017")); MongoDatabase database = mongoClient.getDatabase("dataset"); MongoCollection<Document> coll = database.getCollection("data"); AggregateIterable<Document> output = coll.aggregate(Arrays.asList( new Document("$group", new Document("_id", "$from").append("sum", new Document("$sum", "$to"))))); for (Document dbObject : output) { System.out.println("_id : " + dbObject.get("_id") + " sum : " + dbObject.get("sum")); }

db.data.aggregate([{

    $group: {

    _id: null,

    count: {

        "$addToSet": "$from"

    }

    }

}, {

    $project: {

    "size": {

        $size: "$count"

    }

    }

}])

AggregateIterable<Document> output = coll .aggregate( Arrays.asList( new Document("$group",

new Document("_id", "null").append("fromCount", new Document("$addToSet", "$from"))), new Document("$project", new Document("size", new Document("$size", "$fromCount"))))); for (Document dbObject : output) { System.out.println(" size : " + dbObject.get("size")); }