nosql - How to sum the value of a key across all documents in a MongoDB collection

Question

I have collection in MongoDB:

{ "_id" : ObjectId("4d2407265ff08824e3000001"), "subida" : 3.95 }
{ "_id" : ObjectId("4d2551b4ae9fa739640df821"), "subida" : 6.03 }
{ "_id" : ObjectId("4d255b115ff08821c2000001"), "subida" : 5.53 }
{ "_id" : ObjectId("4d25e8d55ff08814f8000001"), "subida" : 1.96 }

How I can sum the value of a key, e.g., "subida", across all documents? With the documents above, I should receive something along the lines of:

{ "subida" : 17.47 }

Answer 1

In this case, aggregation is much simpler and much more efficient than mapReduce:

db.collection.aggregate({
    $group: {
        _id: '',
        subida: { $sum: '$subida' }
    }
 }, {
    $project: {
        _id: 0,
        subida: '$subida'
    }
})

1. use $group with $sum to calculate sum
2. use projection's $project operator to remove id key required by $group operator