How to get aggregation of child collection in mongodb

Question

I have a data structure like this:

{
  "_id": "order1",
  "transactions": [
    {
      "amount": 100,
      "type": "payment"
    },
    {
      "amount": -10,
      "type": "refund"
    }
  ]
}

I want to get sum of amount which is 100+(-10) = 90. I am new to mongodb. Can someone help me write query.

Answer 1

you can use aggregate with $unwind

.aggregate([
{$unwind:"$transactions"},
{$group: {_id:"$_id", total: {$sum: "$transactions.amount"}}}
])

Answer 2

> db.aaa.aggregate([{$unwind:"$transactions"},{$group:{_id:null, total:{$sum:"$transactions.amount"}}}])
{ "_id" : null, "total" : 90 }