using aggregation in mongodb

Question

I have following records in mongodatabase.

> db.student.find()
{ "_id" : ObjectId("52ca76140e468ba197e50c23"), "name" : "pratik", "subject" : "maths", "marks" : 68 }
{ "_id" : ObjectId("52ca762b0e468ba197e50c24"), "name" : "pratik", "subject" : "biology", "marks" : 96 }
{ "_id" : ObjectId("52ca77a90e468ba197e50c25"), "name" : "pratik", "subject" : "maths", "marks" : 40 }

From this record I want to know the total marks obtained for just maths subject. Here is what I have tried,but I don't know what is going wrong in the following query.

db.student.aggregate(
{$match: { 'subject': "maths"}},
{ $group : { _id :{ name:"$name",subject:"$subject",marks:"$marks" },
            total: { $sum : "$marks"}}
})

{
        "result" : [
                {
                        "_id" : {
                                "name" : "pratik",
                                "subject" : "maths",
                                "marks" : 40
                        },
                        "total" : 40
                },
                {
                        "_id" : {
                                "name" : "pratik",
                                "subject" : "maths",
                                "marks" : 68
                        },
                        "total" : 68
                }
        ],
        "ok" : 1
}

Could you please let me know what has went wrong in the above query along with the correct answers. Also suggest me the appropriate guide to use the aggregation module so that I can use it efficiently.I am beginner to aggregation module of mongo database.

Thanks in Advance.

Sammaye · Accepted Answer · 2014-01-06 09:55:47Z

1

The problem is the marks field in the group within the _id. It will group upon mark scores then which is useless to you, instead you want:

db.student.aggregate(
    {$match: { 'subject': "maths"}}, 
    {$group : { 
        _id :{ name:"$name", subject:"$subject" }, 
        total: { $sum : "$marks"} 
    }}
);

answered Jan 6, 2014 at 9:55

Sammaye

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2 Comments

user1961408 Over a year ago

This has worked for me. Thanks Sammaye.Could you please suggest me the appropriate guide to use the aggregation module so that I can use it efficiently.I am beginner to aggregation module of mongo database and bit confused.

Sammaye Over a year ago

@user1961408 A good one is normally the zip code example: docs.mongodb.org/manual/tutorial/aggregation-zip-code-data-set it should teach you most things

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