$group with $map not working properly in mongodb - mongodb

Question

I am new to mongodb aggregation, i have a collection
{
"_id":60ab3312623b0108338a9601,
"items":[{"type":"Tshirt","price":300,"quantity":2}],
"orderId":"ORD189",
"from":"abc",
"to":"xyz",
"createdAt":2021-05-24T05:01:06.960Z,
"__v":0,
"tracking":null
}
I want to find the count of orders per day and totalPrice of items like
{_id: {date:24, month:05, year:2021}, count: 1, totalPrice: 600}
I used aggregations like below
db.getCollection("orders").aggregate(
[
{
"$group" : {
"_id" : {
"date" : {
"$dayOfMonth" : "$createdAt"
},
"month" : {
"$month" : "$createdAt"
},
"year" : {
"$year" : "$createdAt"
}
},
"count" : { $sum : 1},
"totalPrice" : {
$sum: {
$map: {
"input" :"$items",
"as":"item",
"in":{
$multiply: ["$item.price", "$item.quantity"]
}
}
}
}
}
}
]
);
It is giving me the following result
{
"_id" : {
"date" : NumberInt(24),
"month" : NumberInt(5),
"year" : NumberInt(2021)
},
"count" : 1.0,
"totalPrice" : NumberInt(0)
}
If I use unwind am not able to get the proper count.

Answer 1

You just need to correct totalPrice calculation,
$map to iterate loop of items and do multiplication of price and quantity, this will return array of total
$sum to get total of above total
"totalPrice": {
$sum: {
$sum: {
$map: {
input: "$items",
in: {
$multiply: ["$$this.price", "$$this.quantity"]
}
}
}
}
}
Playground