Mongo DB Aggregations - node.js

Question

I am trying to generate data for a heat map on my dashboard. I need to group the data by days i.e Monday, Tuesday e.t.c and then group the data by time intervals. I am able to group the data by days. But the time interval part doesnt work. I am using MongoDB.
Sample of my Schema is shown below
{
"createdAt": "2020-06-30T22:47:32+00:00",
"day": "Wednesday",
"user": {
"$oid": "5ec51d59ddfb380017649591"
},
"country": "NG",
"city": "",
"userType": "superadmin",
"fullName": "Jane Doe",
"__v": 0
}
This is what I could come up with so far
let heatMap = await await Session.aggregate([
{
$group: {
_id: "$day",
count: { $sum: 1 },
},
},
]);
This groups by day for me already.
I need to further group by intervals of hours
Sample Expected Data
[
{
_id: "Sunday",
intervals: [
{
interval1: "12am-6am",
count: 34,
},
{
interval2: "6am-12noon",
count: 44,
},
],
},
{
_id: "Monday",
intervals: [
{
interval1: "12am-6am",
count: 34,
},
{
interval2: "6am-12noon",
count: 44,
},
],
},
];

Answer 1

You can use this pipeline that utilizes $mod with some date operators to achieve the result you want.
The strategy is first to group by the intervals and the finally by the day of the week as this simplifies the process.
db.collection.aggregate([
{
"$group": {
"_id": {
"$toDate": {
"$subtract": [
{
"$toLong": {
$toDate: "$createdAt"
}
},
{
"$mod": [
{
"$toLong": {
$toDate: "$createdAt"
}
},
21600000
]
}
]
}
},
count: {
$sum: 1
}
}
},
{
$group: {
_id: {
day: {
$dayOfWeek: "$_id"
},
hour: {
$hour: "$_id"
}
},
count: {
$sum: "$count"
},
interval: {
$first: "$_id"
}
}
},
{
$group: {
_id: "$_id.day",
intervals: {
$push: {
count: "$count",
interval: "$interval"
}
}
}
}
])
Notice that the result is not exactly how you want, i.e _id: 3 is equal to _id: "Tuesday" As Mongo does not provide any out of the box conversions you'll have to do it yourself either in code or by using something like $switch
Mongo Playground