MongoDB Sum-Query does not sum - mongodb

Question

I have the following documents:
{ "_id" : ObjectId("5d9db4462034bf17454d7d33"), "name" : "Product1", "cost_oneoff" : "1", "cost_monthly" : "1", "margin_oneoff" : "1", "margin_monthly" : "1", "price_oneoff" : "1", "price_monthly" : "1" }
{ "_id" : ObjectId("5d9dc2f2d8e17309b46f9b03"), "name" : "Product2", "cost_oneoff" : "0", "cost_monthly" : "1", "margin_oneoff" : "0,5", "margin_monthly" : "0,5", "price_oneoff" : "0", "price_monthly" : "2" }
I want the sum of e.g. cost monthly with the following statement:
{ "_id" : null, "total" : 0 }
Can someone help me?
db.service_items.aggregate([
{ $match: {$or: [{"_id": ObjectId("5d9db4462034bf17454d7d33")},{"_id": ObjectId("5d9dc2f2d8e17309b46f9b03")}]}},
{ $group:
{_id: null,
total: {
$sum: "$cost_monthly"
}
}
}
])
Result:
{ "_id" : null, "total" : 0 }
The desired answer is 2

Answer 1

The $sum operator only works on an integer. According to the docs it ignores non-numeric values. You seem to have them stored as a string. Change cost_monthly to an integer and you should get the desired result:
"cost_monthly" : 1
You can check it out here.

Answer 2

Like #silencedogood said, The $sum operator only works on an integer. We need to convert the string to a numeric value using $toInt operator.
The following is an example:
db.service_items.aggregate([
{
$match: {
$or: [
{
"_id": ObjectId("5d9db4462034bf17454d7d33")
},
{
"_id": ObjectId("5d9dc2f2d8e17309b46f9b03")
}
]
}
},
{
$group: {
"_id": null,
"total": {
$sum: {
$toInt: "$cost_monthly"
}
}
}
}
])
Note: The $toInt is introduced in Mongo v4.0