mongodb multiple aggregations in single operation

Question

I have an item collection with following documents.

{ "item" : "i1", "category" : "c1", "brand" : "b1" }  
{ "item" : "i2", "category" : "c2", "brand" : "b1" }  
{ "item" : "i3", "category" : "c1", "brand" : "b2" }  
{ "item" : "i4", "category" : "c2", "brand" : "b1" }  
{ "item" : "i5", "category" : "c1", "brand" : "b2" }  

I want to separate aggregation results --> count by category, count by brand. Please note, it is not count by (category,brand)

I am able to do this using map-reduce using following code.

map = function(){
    emit({type:"category",category:this.category},1);
    emit({type:"brand",brand:this.brand},1);
}
reduce = function(key, values){
    return Array.sum(values)
}
db.item.mapReduce(map,reduce,{out:{inline:1}})

And the result is

{
        "results" : [
                {
                        "_id" : {
                                "type" : "brand",
                                "brand" : "b1"
                        },
                        "value" : 3
                },
                {
                        "_id" : {
                                "type" : "brand",
                                "brand" : "b2"
                        },
                        "value" : 2
                },
                {
                        "_id" : {
                                "type" : "category",
                                "category" : "c1"
                        },
                        "value" : 3
                },
                {
                        "_id" : {
                                "type" : "category",
                                "category" : "c2"
                        },
                        "value" : 2
                }
        ],
        "timeMillis" : 21,
        "counts" : {
                "input" : 5,
                "emit" : 10,
                "reduce" : 4,
                "output" : 4
        },
        "ok" : 1,
}

I can get same results by firing two different aggregation commands as below.

db.item.aggregate({$group:{_id:"$category",count:{$sum:1}}})
db.item.aggregate({$group:{_id:"$brand",count:{$sum:1}}})

Is there anyway I can do the same using aggregation framework by single aggregation command.

I have simplified my case here, but in actual I need this grouping from fields in array of subdocuments. Assume the above is structure after I do unwind.

It is a real-time query (someone waiting for response), though on smaller dataset, so execution time is important.

I am using MongoDB 2.4.

Answer 1

Starting in Mongo 3.4, the $facet aggregation stage greatly simplifies this type of use case by processing multiple aggregation pipelines within a single stage on the same set of input documents:

// { "item" : "i1", "category" : "c1", "brand" : "b1" }
// { "item" : "i2", "category" : "c2", "brand" : "b1" }
// { "item" : "i3", "category" : "c1", "brand" : "b2" }
// { "item" : "i4", "category" : "c2", "brand" : "b1" }
// { "item" : "i5", "category" : "c1", "brand" : "b2" }
db.collection.aggregate(
  { $facet: {
      categories: [{ $group: { _id: "$category", count: { "$sum": 1 } } }],
      brands:     [{ $group: { _id: "$brand",    count: { "$sum": 1 } } }]
  }}
)
// {
//   "categories" : [
//     { "_id" : "c1", "count" : 3 },
//     { "_id" : "c2", "count" : 2 }
//   ],
//   "brands" : [
//     { "_id" : "b1", "count" : 3 },
//     { "_id" : "b2", "count" : 2 }
//   ]
// }