Calculate discount in mongodb - mongodb

Question

I'd like to post some products to the body and get back the calculated total amount.
But it's getting complicated for me when need to apply discount in that form:
for every $amount of $product the price reduced to $new-price
(let's say every banana is 1$, if customer buy 3 then price is 2$ (but they can buy as many..))
How can I achieve that?

Answer 1

data
db={
"orders": [
{
"_id": "1",
"customer_id": "1",
"items": [
{
"product_id": "1",
"quantity": 2
},
{
"product_id": "2",
"quantity": 5
}
]
}
],
"product": [
{
"product_id": "1",
"name": "apple",
"price": 2,
"quantity": 1,
"free": 0
},
{
"product_id": "2",
"name": "banana",
"price": 1,
"quantity": 3,
"free": 1
}
]
}
aggregate
db.orders.aggregate([
{
"$match": {
_id: "1"
}
},
{
"$unwind": "$items"
},
{
"$lookup": {
"from": "product",
"localField": "items.product_id",
"foreignField": "product_id",
"as": "product_docs"
}
},
{
"$set": {
"product_doc": {
"$first": "$product_docs"
}
}
},
{
"$project": {
"total_each": {
"$multiply": [
{
$subtract: [
"$items.quantity",
{
"$multiply": [
{
$floor: {
$divide: [
"$items.quantity",
"$product_doc.quantity"
]
}
},
"$product_doc.free"
]
}
]
},
"$product_doc.price"
]
}
}
},
{
"$group": {
"_id": "$_id",
"total": {
"$sum": "$total_each"
}
}
}
])
result:
apple no discount, banana buy 3 get 1 free
2x2 + {5-[floor(5/3)x1]}x1 = 8
[
{
"_id": "1",
"total": 8
}
]
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