svs teja February 2016

How to give Multiple Projections in MongoDB java using Aggregation Framework

i have three documents

{ "_id" : 3, "quizzes" : [ 4, 5, 5 ], "labs" : [ 6, 5 ], "final" : 78, "midterm" : 70 }
{ "_id" : 2, "quizzes" : [ 9, 10 ], "labs" : [ 8, 8 ], "final" : 95, "midterm" : 80 }
{ "_id" : 1, "quizzes" : [ 10, 6, 7 ], "labs" : [ 5, 8 ], "final" : 80, "midterm" : 75, "extraMarks" : 10 }

I have to query exam total fields as sum of final +midterm+extramarks. The query is as follows: Query1:

  db.students.aggregate([ { "$project": { "final": 1, "midterm": 1, "examTotal": { "$add": [ "$final", "$midterm", { "$ifNull": [ "$extraMarks", 10 ] } ] } } } ])

But along with this i need to get this only for people whose id is greater than equal to 1 and less than equal to 2

So i have modified the code to the following: Query2:

 db.students.aggregate([ { $match: { $and: [ { _id: { $gte: 1, $lte: 2 } }]}}, { "$project": { "final": 1, "midterm": 1, "examTotal": { "$add": [ "$final", "$midterm", { "$ifNull": [ "$extraMarks", 10 ] } ] } } } ])

How to convert this code to Java code for query 1 i made the code and is working? How to add match stage in the existing code which is as follows:

BsonArray fields=new BsonArray();
            BsonArray defaultValue1=new BsonArray();
            defaultValue1.add(new BsonString("$extraMarks"));
            defaultValue1.add(new BsonDouble(0d));          
            BsonDocument ifNullProjection=new BsonDocument();
            ifNullProjection.put("$ifNull",defaultValue1);
            fields.add(new BsonString("$final"));
            fields.add("$midterm");
            fields.add(ifNullProjection);
            BsonDocument addObject=new BsonDocument();
            addObject.append("$add", fields);
            BsonDocument valueTobeUpdate=new BsonDocument();
            valueTobeUpdate.append("sum", addObject);
            BsonDocument mainProjection=new BsonDocument        

Answers


chridam February 2016

The $and operator in the $match pipeline is not really necessary since you can implicitly do an AND operation by just specifying a comma separated list of expressions.

The aggregation pipeline can be restructured to :

Mongo shell:


/*
    MONGO SHELL: 
    var pipeline = [ 
        { 
            "$match": { "_id": { "$gte": 1, "$lte": 2 } } // or  "$match": { "_id": { "$in": [1,2] } }
        }, 
        { 
            "$project": { 
                "final": 1, 
                "midterm": 1, 
                "examTotal": { 
                    "$add": [ "$final", "$midterm", { "$ifNull": [ "$extraMarks", 10 ] } ] 
                } 
            } 
        } 
    ];
    db.students.aggregate(pipeline);

*/

Java implementation:

public class JavaAggregation {
    public static void main(String args[]) throws UnknownHostException {

        MongoClient mongo = new MongoClient();
        DB db = mongo.getDB("test");

        DBCollection coll = db.getCollection("students");

        // create the pipeline operations, first with the $match
        DBObject match = new BasicDBObject("$match",
                            new BasicDBObject("_id",
                                new BasicDBObject("$gte", 1).append("$lt", 2)
                            )
                        );

        // build the $project operations
        BasicDBList coalesce = new BasicDBList();
        coalesce.add("$extraMarks");
        coalesce.add(10)
        DBObject ifNullClause = new BasicDBObject("$ifNull", coalesce);  

        BasicDBList addition = new BasicDBList();
        addition.add("$final");
        addition.a 

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Asked in February 2016
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