Sum in nested document MongoDB

Question

I\'m trying to sum some values in an array of documents, with no luck.

This is the Document

db.Cuentas.find().pretty()

{
    \"Agno\": \"2013         


        

Answer 1

As Sammaye indicated, you need to $unwind the Egresos array to duplicate the matched doc per array element so you can $sum over each element:

db.Cuentas.aggregate([
    {$match: {"Usuario": "MarioCares"} }, 
    {$unwind: '$Egresos'}, 
    {$group: {
        _id: null, 
        "suma": {$sum: "$Egresos.Monto" }
    }}
])

Answer 2

You can do also by this way. don't need to group just project your fields.

db.Cuentas.aggregate([
    { $match: { "Usuario": "MarioCares" } },
    {
        $project: {
            'MontoSum': { $sum: "$Egresos.Monto" }
        }
    }
])