Find by biggest difference between elements on embedded object array - mongodb

Question

Given a list of products like this:
{
"_id" : ObjectId("5a594f8eff9da13c9d415a63"),
"productId" : "xxx",
"date" : "2018-09-13",
"prices" : [
{
"country" : "en",
"price" : 16.5,
"currency" : "EUR"
},
{
"country" : "es",
"price" : 17.78,
"currency" : "EUR"
},
{
"country" : "fr",
"price" : 18.08,
"currency" : "EUR"
},
{
"country" : "de",
"price" : 18.89,
"currency" : "EUR"
},
{
"country" : "it",
"price" : 27.49,
"currency" : "EUR"
}
]
}
Given a country code and a date, is there any way to find the products for that date and order by biggest different between price for the country?
Thank you very much in advance

Answer 1

Assuming that
you want the biggest difference between the given country and any other country and
there are no duplicate product ids (if there are, the latest product will be used, thanks to this line "$last": "$prices"),
try this:
db.collection.aggregate([
{
"$match": {
"date": "2018-09-13" // replace with date variable
}
},
{
"$group": {
"_id": "$productId",
"prices": {
"$last": "$prices"
}
}
},
{
"$addFields": {
"pricesObj": {
"$map": {
"input": "$prices",
"in": {
"k": "$$this.country",
"v": "$$this.price"
}
}
}
}
},
{
"$addFields": {
"pricesObj": {
"$arrayToObject": "$pricesObj"
}
}
},
{
"$addFields": {
"reference": "$pricesObj.es" // replace with country variable
}
},
{
"$addFields": {
"differences": {
"$map": {
"input": "$prices",
"in": {
"country": "$$this.country",
"difference": {
"$abs": {
"$subtract": [
"$$this.price",
"$reference"
]
}
}
}
}
}
}
},
{
"$addFields": {
"biggestDifference": {
"$reduce": {
"input": "$differences",
"initialValue": {
difference: 0
},
"in": {
"$cond": [
{
"$gt": [
"$$this.difference",
"$$value.difference"
]
},
"$$this",
"$$value"
]
}
}
}
}
},
{
"$project": {
"_id": 1,
"biggestDifference": "$biggestDifference.difference"
}
},
{
"$sort": {
"biggestDifference": -1
}
}
])
I'm sure it could be expressed more concisely, but it works: https://mongoplayground.net/p/y67jhhFBB9l
The output looks like:
[
{
"_id": "xxy",
"biggestDifference": 12295.109999999999
},
{
"_id": "xxx",
"biggestDifference": 98.72
}
]
for this input:
[
{
"productId": "xxx",
"date": "2018-09-13",
"prices": [
{
"country": "en",
"price": 116.5,
"currency": "EUR"
},
{
"country": "es",
"price": 17.78,
"currency": "EUR"
},
{
"country": "fr",
"price": 18.08,
"currency": "EUR"
},
{
"country": "de",
"price": 18.89,
"currency": "EUR"
},
{
"country": "it",
"price": 27.49,
"currency": "EUR"
}
]
},
{
"productId": "xxy",
"date": "2018-09-13",
"prices": [
{
"country": "en",
"price": 16.5,
"currency": "EUR"
},
{
"country": "es",
"price": 17.78,
"currency": "EUR"
},
{
"country": "fr",
"price": 18.08,
"currency": "EUR"
},
{
"country": "de",
"price": 12312.89,
"currency": "EUR"
},
{
"country": "it",
"price": 997.49,
"currency": "EUR"
}
]
}
]

Answer 2

Thank you #jaksz,
Finally, I'm using this approach that works like a charm (because the smaller price is always in the first position of the array):
db.productPrices.aggregate(
[
{
"$match": {
"date": "2018-09-13" // replace with date variable
}
},
{
"$group": {
"_id": "$productId",
"prices": {
"$last": "$prices"
}
}
},
{
"$addFields": {
"pricesObj": {
"$map": {
"input": "$prices",
"in": {
"k": "$$this.country",
"v": "$$this.price"
}
}
}
}
},
{
"$addFields": {
"pricesObj": {
"$arrayToObject": "$pricesObj"
}
}
},
{
"$addFields": {
"reference": "$pricesObj.es" // replace with country variable
}
},
{
"$addFields": {
"cheapest": {
"$arrayElemAt": ["$prices", 0]
}
}
},
{
"$addFields": {
"difference": {
"$abs": {
"$subtract": ["$reference", "$cheapest.price"]
}
}
}
},
{
"$project": {
"_id": 1,
"prices": "$prices",
"difference": "$difference"
}
},
{
"$sort": {
"difference": -1
}
}
]).pretty()