Group documents with MongoDB - mongodb

Question

I come from the world of relational databases and am having difficulty with a MongoDB query.
In my database I have telemetry documents with minute-by-minute records for each monitored system.
I'm tending to do a query that groups records by hour.
Below is a demonstration of the structure of my documents:
{
_id: ObjectId("61847b83618fc8a6fb368ab7"),
system_code: 22,
date: ISODate("2021-08-01T00:00:01.000Z"),
value1: 22.2372973251,
value2: 20
}
Here's the structure of the query I'm trying to run:
db.telemetry.aggregate([
{
$match: {
$and: [
{ system_code: 22 },
{ date: { $gte: ISODate("2021-08-01 00:00:00") } },
{ date: { $lte: ISODate("2021-08-01 02:00:00") } }
]
}
},
{
$addFields: {
italianDate: {
$dateToString: {
format: "%d/%m/%Y %H:00:00",
date: "$date"
}
}
}
},
{
$sort: {
italianDate: 1
}
},
{
$group: {
_id: {
year: { $year: "$date" },
month: { $month: "$date" },
day: { $dayOfMonth: "$date" },
hour: { $hour: "$date" }
}
}
}
]);
The SQL query with PostgreSQL is exactly this:
SELECT TO_CHAR(t.date, 'YYYY-MM-DD HH24:00:00') AS italianDate, AVG(value1), AVG(value2) FROM telemetry t WHERE t.system_code = 22 AND t.date BETWEEN '2021-08-01 00:00:00' AND '2021-08-01 02:00:00' GROUP BY italianDate ORDER BY italianDate ASC
Thank you so much if you can help me.

Answer 1

You are actually pretty close. You just need to put the $avg inside your $group stage. Another thing is you need to pay attention to the date formats. Refer to this official MongoDB document for the formats.
db.telemetry.aggregate([
{
$match: {
system_code: 22,
date: {
$gte: ISODate("2021-08-01T00:00:00Z"),
$lte: ISODate("2021-08-01T02:00:00Z")
}
}
},
{
$addFields: {
italianDate: {
$dateToString: {
format: "%Y-%m-%d %H:00:00",
date: "$date"
}
}
}
},
{
$group: {
_id: "$italianDate",
avgValue1: {
$avg: "$value1"
},
avgValue2: {
$avg: "$value2"
}
}
},
{
$sort: {
_id: -1
}
}
])
Here is the Mongo playground for your reference.