MongoDB and Aggregation Framework

Question

I have the following collection in mongodb.

{ "_id" : ObjectId("519a35ee8f2ceda43f42add5"), "articulo" : "Sobre mongodb", "autor" : "xxxx1", "calificacion" : 3 }
{ "_id" : ObjectId("519a360b8f2ceda43f42add6"), "articulo" : "Aggregation framework", "autor" : "xxxx1", "calificacion" : 5 }
{ "_id" : ObjectId("519a361b8f2ceda43f42add7"), "articulo" : "Sobre journal", "autor" : "xxxx2", "calificacion" : 4 }
{ "_id" : ObjectId("519a362e8f2ceda43f42add8"), "articulo" : "Manipulando datos", "autor" : "xxxx1", "calificacion" : 2 }
{ "_id" : ObjectId("519a36418f2ceda43f42add9"), "articulo" : "MongoDB for dba", "autor" : "xxxx2", "calificacion" : 5 }
{ "_id" : ObjectId("519a4aa18f2ceda43f42adda"), "articulo" : "ejemplo2", "autor" : "xxxx1", "calificacion" : 5 }

I want to count the number of the articles (articulos) with max grade (calificacion) by author(autor).

xxxx1 has 2 articles with grade of 5 xxxx2 has 1 articles with grade of 5

(I don't know what's the max grade)

I've tried this:

db.ejemplo.aggregate([
    {$group:{_id: "$autor" , calificacion:{$max:"$calificacion" }}} 
])

but I only get the authors with max grade. Could I do it with Aggregation Framework?

Linda Qin · Accepted Answer · 2013-05-21 06:28:33Z

2

You can try the aggregation operation like this:

db.ejemplo.aggregate([
    { $group : { _id : { autor : "$autor",
                         calificacion : "$calificacion" },
                 articulos : { $sum : 1 },
    }},
    { $sort : { "_id.calificacion" : -1 }},
    { $group : { _id : "$_id.autor",
                 calificacion : { $first : "$_id.calificacion" },
                 articulos : { $first : "$articulos" },
    }}
])

And the result is like this:

{
    "result" : [
        {
            "_id" : "xxxx1",
            "calificacion" : 5,
            "articulos" : 2
        },
        {
            "_id" : "xxxx2",
            "calificacion" : 5,
            "articulos" : 1
        }
    ],
    "ok" : 1
}

Thanks, Linda

answered May 21, 2013 at 6:28

Linda Qin

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1 Comment

drsromero Over a year ago

Ohh Thank you!!!! It works!!!! I'm learning mongo so I need to understand how I can do simples queries.

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