How do we calculate a monthly average value in MongoDB? - mongodb

Question

I have a list of documents in Mongo, each containing the value of a metric for a service for that day. The scheme is as follows:
_id: ObjectId("..."),
name: '<some_metric_name>',
service: '<service_name>',
timestamp_unix: Long("1635033600000"),
timestamp: '2021-10-24 03:00:00 +0300 EEST',
value: 99.9810785241248
},
I would like to have a dashboard in Redash where I can see the calculated average of these values per month, grouped by service.
e.g. over a span of 4 months it should look like this:
| service | date | avg
| my_svc | 01-01-2022 to 31-01-2022 | 99.500
| my_svc | 01-02-2022 to 28-02-2022 | 99.100
| my_svc | 01-03-2022 to 31-03-2022 | 99.400
| my_svc | 01-04-2022 to 30-04-2022 | 99.900
| my_svc_total | 01-01-2022 to 30-04-2022 | 99.475
| my_svc_2 | 01-01-2022 to 31-01-2022 | 99.150
.
.
.
So I need a query that is going to aggregate the documents by month, calculate the average and group them by month and service. So far I have this that can calculate the overall average:
{
"collection": "metrics",
"aggregate": [
{
"$group": {
"_id": {
"$dateToString": {
"format": "%Y-%m-%d",
"date": {
"$toDate": "$timestamp_unix"
}
}
},
"total_average": {
"$avg": "$value"
}
}
}
]
}
So how would I go about implementing this?

Answer 1

I think you want your data group by month+year and service. So you should put both condition in your group stage. Then, you can get the average with the accumulator as you did.
{
"collection": "metrics",
"aggregate": [
{
"$group": {
"_id": {
"service" : "$service",
"month" :
{ "$dateToString": {
"format": "%Y-%m",
"date": {
"$toDate": "$timestamp_unix"
}
}
}
},
"average": {
"$avg": "$value"
}
}
},
{ "$project" :
{"service" : "$_id.service",
"year-month":"$_id.month",
"average":1,
"_id":0
}
},
{ "$sort": {"service" : 1, "year-month" : 1} }
]
}
This should give you data you want.
If you also want the total average per service, i would suggest to do use a $facet stage and do the exact same group stage but without the date condition.