How to use “$avg” aggregate expression in projection stage with mongodb java driver?

[zeinab sheikhi]

Hi.

I have a collection in mongodb named Buffer in which documents are like this:

{ "load" : { "metric" : { "behind" : 35, "side" : 15, "top" : 135, "front" : 0 } } }

{ "load" : { "metric" : { "behind" : 35, "side" : 76, "top" : 120, "front" : 13 } } }

{ "load" : { "metric" : { "behind" : 47, "side" : 122, "top" : 0, "front" : 25 } } }

{ "load" : { "metric" : { "behind" : 67, "side" : 88, "top" : 11, "front" : 38 } } }

{ "load" : { "metric" : { "behind" : 100, "side" : 9, "top" : 42, "front" : 41 } } }

{ "load" : { "metric" : { "behind" : 119, "side" : 56, "top" : 12, "front" : 59 } } }

{ "load" : { "metric" : { "behind" : 131, "side" : 90, "top" : 34, "front" : 62 } } }

{ "load" : { "metric" : { "behind" : 165, "side" : 145, "top" : 56, "front" : 176 } } }

I wanted to use aggregation and calculate the average of side and top values. also I want to calculate the average of behind and front value together ,I mean calculating sum of front and behind value and then based on the that calculate average of these two fields. I got this working in the mongo shell and the command is as follow:

db.Buffer.aggregate([{$group:{"_id":null,"avgOfTop":{"$avg":"$load.metric.top"},"avgOfFront":{"$sum":"$load.metric.front"},"avgOfBehind":{"$sum":"$load.metric.behind"},"avgOfSide":{"$avg":"$load.metric.side"}}},{$project:{avgOfBackAndForth:{$avg:["$avgOfFront","$avgOfBehind"]},avgOfTop:1,avgOfSide:1,_id:0}}])

and the result is: { "avgOfTop" : 51.25, "avgOfSide" : 75.125, "avgOfBackAndForth" : 556.5 }

but I don't know how to do avg expression in the projection stage in java.

my java code:

MongoCollection<Document> collection = MongoConnectionManager.getInstance().getMongoCollection();
         final List<BsonField> bsonFieldList = new ArrayList<>();
        bsonFieldList.add(avg("averageOfTop","$load.metric.top"));
        bsonFieldList.add(avg("averageOfSide","$load.metric.side"));
        bsonFieldList.add(sum("sumOfFront","$load.metric.front"));
        bsonFieldList.add(sum("sumOfBehind","$load.metric.behind"));

        Bson match = match(gte(ConstantStrings.ID_KEY,0));
        Bson group = group(null,bsonFieldList);
        Bson project = project(fields(excludeId(),
                include(ConstantStrings.AVERAGE_OF_TOP_METRICS,ConstantStrings.AVERAGE_OF_SIDE_METRICS)
               // computed(ConstantStrings.AVERAGE_OF_BACK_AND_FORTH_METRICS,query)
                ));

        AggregateIterable<Document> rs = collection.aggregate(Arrays.asList(match,group,project));
        for (Document dbObject : rs)
        {
            System.out.println(dbObject.toJson());
        }

the results is : {"averageOfTop": 51.25, "averageOfSide": 75.125}

MongoDB documentation for java driver states that: "For $group accumulator expressions, the Java driver provides Accumulators helper class. For other aggregation expressions, manually build the expression Document."

but I don't know how to use compute() to build {avgOfBackAndForth:{$avg:["$avgOfFront","$avgOfBehind"]}
any help would be appreciated.

[Prasad Saya]

There is a simple way to create your Java  code for MongoDB Aggregation queries. The MongoDB Compass (a GUI desktop tool) has the features to create the native (or shell) Aggregation query and then export it to your favorite programming language (Java, Python, etc.). Here are the links to the Compass docs about the usage:

Aggregation Pipeline Builder

Export Pipeline to Specific Language