Sum columns mongodb

I have this collection

{a:0,b:1,c:1,d:1}
{a:1,b:1,c:0,d:1}
{a:1,b:1,c:0,d:1}

and I want this result with mapReduce

{id: any, A:2,B:3,C:1,D:3}

but I don't know how do it, I only can sum onde column

how i do this?

You could emit a common key for all the records, in the map function:

var map = function(){emit(1,this);}

The reduce function would calculate the sum and return the values.

var reduce = function(key,values){
var result = {};
values.forEach(function(i){
 Object.keys(i).forEach(function(key){
 if(result.hasOwnProperty(key)){
    result[key] = i[key]+result[key];
 }
 else{
    result[key] = i[key];
 }
})
})
return result;
}

Execute the map reduce:

db.collection.mapReduce(map,reduce,{ out: "out"})

If you knew the keys before hand, then you could easily aggregate the results, using just the $group stage in the pipeline.

db.collection.aggregate([
{$group:{"_id":null,
         "a":{$sum:"$a"},
         "b":{$sum:"$b"},
         "c":{$sum:"$c"},
         "d":{$sum:"$d"}}}
])