How to group by date and by specific field in MongoDB - mongodb

Question

I want to print grouped by date and by "productId" within the date. In this example, the output should be as follow:
[
{
"_id": "2018-03-04",
"product1": 2,
"product2": 2
}
]
Data: https://mongoplayground.net/p/gzvm11EIPn2
How to make it?

Answer 1

When you use the $group stage in aggregation you learn to group by one field as such: { $group: { "_id": "$field1"...
When you want to group by two or more fields "_id" needs to be a subdocument and you pass the fields as key value pairs inside the subdocument as such:
db.mycollection.aggregate([
{
$group:
{
"_id": { "product1": "$product1", "product2": "$product2", ... }
}
}
])
... etc.

Answer 2

$group - Group by createdAt (date string) and productId and perform count via $sum.
$group - Group by createdAtand push data from (1) to products array field.
$replaceRoot - Replace input document with new document.
3.1. $arrayToObject - Convert the object from products array field to key value pair with productId (key) and count (value).
3.2. $mergeObjects - Create object with _id and merge the object from (3.2) into 1 object.
db.collection.aggregate([
{
$group: {
_id: {
createdAt: {
$dateToString: {
format: "%Y-%m-%d",
date: "$createdAt"
}
},
productId: "$productId"
},
count: {
$sum: 1
}
}
},
{
$group: {
_id: "$_id.createdAt",
products: {
$push: {
productId: "$_id.productId",
count: "$count"
}
}
}
},
{
"$replaceRoot": {
"newRoot": {
"$mergeObjects": [
{
_id: "$_id"
},
{
$arrayToObject: {
$map: {
input: "$products",
in: {
k: {
$toString: "$$this.productId"
},
v: "$$this.count"
}
}
}
}
]
}
}
}
])
Sample Mongo Playground
Output
[
{
"5e345223b3aa703b8a9a4f34": 2,
"5e345223b3aa703b8a9a4f35": 2,
"_id": "2018-03-04"
}
]