MongdDB: Combining query results of two collections as one - mongodb

Question

There are two collections (view and click) like following:
# View collection
_id publisher_id created_at
617f8ea98e0f54f05e10e796 1 2021-11-01T00:00:00.000Z
617f8eab8e0f54f05e10e798 1 2021-11-01T00:00:00.000Z
617f8eac8e0f54f05e10e79a 1 2021-11-01T00:00:00.000Z
617f90cea187d30ebbecdee9 2 2021-11-01T00:00:00.000Z
# Click collection
_id publisher_id created_at
617f8ea98e0f54f05e10e796 1 2021-11-01T00:00:00.000Z
617f8eab8e0f54f05e10e798 2 2021-11-01T00:00:00.000Z
How can I get the following expected results with one query?
(or)
What is the best way for the following expected results?
# Expected For Publisher ID(1)
_id view_count click_count
2021/11/1 3 1
# Expected For Publisher ID(2)
_id view_count click_count
2021/11/1 1 1
Currently, I am using 2 queries for both collections and combining results as one in code.
For View
db.view.aggregate([
/*FirstStage*/
{
$match:
{
"$and":
[
{
"publisher_id": 1
},
{
"created_at": {$gte: new ISODate("2021-11-01"), $lt: new ISODate("2021-11-28")}
}
]
}
},
/*SecondStage*/
{
$group:
{
_id: {$dateToString: {format: '%Y/%m/%d', date: "$created_at"}},
count: {
$sum: 1
}
}
}
])
For Click
db.click.aggregate([
/*FirstStage*/
{
$match:
{
"$and":
[
{
"publisher_id": 1
},
{
"created_at": {$gte: new ISODate("2021-11-01"), $lt: new ISODate("2021-11-28")}
}
]
}
},
/*SecondStage*/
{
$group:
{
_id: {$dateToString: {format: '%Y/%m/%d', date: "$created_at"}},
count: {
$sum: 1
}
}
}
])

Answer 1

Because you are querying two different collections there is no "good" way to merge this into one query, the only way I can think of is using $facet, where the first stage is the "normal" one, and the other stage starts with a $lookup from the other collection.
This approach does add overhead, which is why I recommend to just keep doing the merge in code, however for the sake of answering here is a sample:
db.view.aggregate([
{
$facet: {
views: [
{
$match: {
"$and": [
{
"publisher_id": 1
},
{
"created_at": {
$gte: ISODate("2021-11-01"),
$lt: ISODate("2021-11-28")
}
}
]
}
},
],
clicks: [
{
$limit: 1
},
{
$lookup: {
from: "click",
let: {},
pipeline: [
{
$match: {
"$and": [
{
"publisher_id": 1
},
{
"created_at": {
$gte: ISODate("2021-11-01"),
$lt: ISODate("2021-11-28")
}
}
]
}
},
],
as: "clicks"
}
},
{
$unwind: "$clicks"
},
{
$replaceRoot: {
newRoot: "$clicks"
}
}
]
}
},
{
$project: {
merged: {
"$concatArrays": [
"$views",
"$clicks"
]
}
}
},
{
$unwind: "$merged"
},
{
$group: {
_id: {
$dateToString: {
format: "%Y/%m/%d",
date: "$merged.created_at"
}
},
count: {
$sum: 1
}
}
}
])
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