MongoDB group results by time interval - mongodb

Question

I have a collection like below.
{
"field1":"value1",
"created_at":"2022-01-01T11:42:01Z"
},
{
"field1":"value2",
"created_at":"2022-01-01T11:22:15Z"
}
I need to group the results by 15 minute time interval and project the results like below from this collection.
[{
"from":"2022-01-01T11:15:00Z",
"to":"2022-01-01T11:30:00Z",
"count":1
},
{
"from":"2022-01-01T11:30:00Z",
"to":"2022-01-01T11:45:00Z",
"count":1
}]
I am able to get the count by 15 minute time interval using the below query. But I want to project from and to dates as well.
db.collection.aggregate([
{ "$group": {
"_id": {
"year": { "$year": "$created_at" },
"dayOfYear": { "$dayOfYear": "$created_at" },
"hour": { "$hour": "$created_at" },
"interval": {
"$subtract": [
{ "$minute": "$created_at" },
{ "$mod": [{ "$minute": "$created_at"}, 15] }
]
}
}},
"count": { "$sum": 1 }
}}
])

Answer 1

You can try an approach,
$dateToParts get parts of the created_at date
$group by year, month, day, hour, and interval as per mod and subtraction calculation and get the total count
to get from and to date from interval you can use $dateFromParts operator, just to add 15 minutes into the date.
db.collection.aggregate([
{
$addFields: {
created_at: { $dateToParts: { date: "$created_at" } }
}
},
{
$group: {
_id: {
year: "$created_at.year",
month: "$created_at.month",
day: "$created_at.day",
hour: "$created_at.hour",
interval: {
$subtract: [
"$created_at.minute",
{ $mod: ["$created_at.minute", 15] }
]
}
},
count: { $sum: 1 }
}
},
{
$project: {
_id: 0,
count: 1,
from: {
$dateFromParts: {
year: "$_id.year",
month: "$_id.month",
day: "$_id.day",
hour: "$_id.hour",
minute: "$_id.interval"
}
},
to: {
$dateFromParts: {
year: "$_id.year",
month: "$_id.month",
day: "$_id.day",
hour: "$_id.hour",
minute: { $add: ["$_id.interval", 15] }
}
}
}
}
])
Playground