Mongo groupby three fields, get unique count for one field - mongodb

Question

I have records stored in my collection as follows:
{
"sessionId" : "f960e3db-838c-42aa-95ce-a807096f7036",
"date" : "12-02-2020",
"hour" : "13",
"month" : "02",
"time" : "13:46:50",
"weekDay" : "Wednesday",
}
I want to group the above records by 'date', 'hour', getting the number of unique'sessionId' per hour. Something like below:
{
"12-02-2020": {
00: 23,//hour:unique number of sessions in that hour
01: 3,
04: 33,
05: 0,
10: 1
},
"13-02-2020": {
00: 2,//hour:unique number of sessions in that hour
03: 33,
09: 23,
05: 6,
10: 1
}
}
Can anyone please formulate the query for the above?

Answer 1

It is often a challenge when you desire dynamic field names and arrays, I found this solution:
db.collection.aggregate([
// group by hour
{
$group: {
_id: { date: "$date", hour: "$hour" },
sessions: { $addToSet: "$sessionId" }
}
},
// count the sessions
{ $set: { sessions: { $size: "$sessions" } } },
// group by day
{
$group: {
_id: "$_id.date",
hour: { $push: "$_id.hour" },
sessions: { $push: "$sessions" }
}
},
// transform result
{ $set: { data: { $range: [0, { $size: "$hour" }] } } },
{
$set: {
data: {
$map: {
input: "$data",
as: "idx",
in: {
k: { $arrayElemAt: ["$hour", "$$idx"] },
v: { $arrayElemAt: ["$sessions", "$$idx"] }
}
}
}
}
},
// transform day and hour values
{ $set: { v: { $arrayToObject: "$data" } } },
{ $project: { data: { k: "$_id", v: "$v" } } },
{ $set: { data: { $arrayToObject: "$data" } } },
{ $replaceRoot: { newRoot: "$data" } }
])
Mongo playground

Answer 2

You can try as below :
db.collection.aggregate([
/** group based on session & date & hour to get unique docs based on session */
{ $group: { _id: { session: "$sessionId", date: "$date", hour: "$hour" } } },
/** group on date & hour & count no.of docs */
{
$group: {
_id: { date: "$_id.date", hour: "$_id.hour" },
count: { $sum: 1 }
}
},
/** transform into entire data of each doc into data field with converting ['k':k..., 'v':v...] into {k,v} */
{
$project: {
data: {
$arrayToObject: [
[
{
k: "$_id.date",
v: { $arrayToObject: [[{ k: "$_id.hour", v: "$count" }]] }
}
]
]
}
}
},
/** replace root of each doc with new root as data */
{
$replaceRoot: {
newRoot: "$data"
}
}
]);
Test : MongoDB-Playground