Get min value from array of object using aggregate and lookup mongodb - mongodb

Question

I have two collections properties and property_prices and the relation is one property many prices. So I am trying to join them and then find min value from property_prices.monthly_unit_price.unit_price. So I could get the Properties with their prices and min unit_price value from entire property pricing.
Property Collection
{
"_id": "1",
"status": "Approved",
"name": "My Property Lake"
}
Property Price Collection where monthly_unit_price have objects from Jan - Dec
{
"property_prices": [
{
"property_id": "1",
"block_id": "ABC",
"monthly_unit_price": [{ "month": "Jan", "unit_price": 100 }, { "month": "Dec", "unit_price": "1200" }],
},
{
"property_id": "1",
"block_id": "DEF",
"monthly_unit_price": [{ "month": "Jan", "unit_price": "200" }, { "month": "Dec", "unit_price": "2400" }],
}
]
}
Basically I want to get the min value from property_prices unit_price for property_id 1
So I tried using aggregate and lookup but I cant get the min value for entire property from property_prices.
Here is what I tried
await Property.aggregate([
{
$lookup: {
from: 'property_prices',
as: 'property_prices',
let: { property_id: '$_id' },
pipeline: [
{
$match: {
$expr: {
$and: [
{ $eq: ['$property_id', '$$property_id'] },
{ $eq: ['$status', 'Completed'] },
]
}
}
},
]
},
},
{
$unwind: "$property_prices"
},
{
$group: {
_id: '$property_prices.property_id',
minInvestment: { "$min": "$property_prices.monthly_unit_price.unit_price" }
}
},
]);
Result I am expecting is
{
"_id": "1",
"status": "Approved",
"name": "My Property Lake",
"property_prices": [
{
"property_id": "1",
"block_id": "ABC",
"monthly_unit_price": [{ "month": "Jan", "unit_price": 100 }, { "month": "Dec", "unit_price": "1200" }],
},
{
"property_id": "1",
"block_id": "DEF",
"monthly_unit_price": [{ "month": "Jan", "unit_price": "200" }, { "month": "Dec", "unit_price": "2400" }],
}
],
"minInvestment":100
}

Answer 1

You are on the right track, you just need to "massage" the document structure a little bit more due to the fact it's a nested array. here is a quick example of doing so using the $map and $reduce operators.
Notice I also had to cast the values to number type using $toInt, I recommend these sort of things to be handled at update/insertion time instead.
db.properties.aggregate([
{
$lookup: {
from: "property_prices",
as: "property_prices",
let: {
property_id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$and: [
{
$eq: [
"$property_id",
"$$property_id"
]
},
{
$eq: [
"$status",
"Completed"
]
}
]
}
}
}
]
}
},
{
$addFields: {
minInvestment: {
$min: {
$reduce: {
input: {
$map: {
input: "$property_prices",
as: "property",
in: {
$map: {
input: "$$property.monthly_unit_price",
as: "price",
in: {
$toInt: "$$price.unit_price"
}
}
}
}
},
initialValue: [],
in: {
"$concatArrays": [
"$$value",
"$$this"
]
}
}
}
}
}
}
])
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