mongodb query to find the min price in array of objects - mongodb

Question

My documents:
[{
"title": "lenovo x-100",
"brand": "lenovo",
"category": "laptops",
"variant": [{
"price": 30000,
"RAM": "4GB",
"storage": "256GB",
"screen": "full hd",
"chip": "i3"
}, {
"price": 35000,
"RAM": "8GB",
"storage": "512GB",
"screen": "full hd",
"chip": "i5"
}, {
"price": 40000,
"RAM": "12GB",
"storage": "2TB",
"screen": "uhd",
"chip": "i7"
}],
"salesCount": 32,
"buysCount": 35,
"viewsCount": 60
},
{
"title": "samsung12",
"brand": "lenovo",
"category": "mobile phones",
"variant": [{
"price": 11000,
"RAM": "4GB",
"ROM": "32GB"
}, {
"price": 16000,
"RAM": "6GB",
"ROM": "64GB"
}, {
"price": 21000,
"RAM": "8GB",
"ROM": "128GB"
}],
"salesCount": 48,
"buysCount": 39,
"viewsCount": 74
}
Expected output
{
_id:"lenovo",
minPrice:1100
}
I have tried this method of aggregation
[{
$match: {
brand: 'lenovo'
}
}, {
$group: {
_id: '$brand',
prices: {
$min: '$variant.price'
}
}
}, {
$unwind: {
path: '$prices'
}
}, {
$group: {
_id: '$_id',
minPrice: {
$min: '$prices'
}
}
}]
I want to find the minimum price based on the brand, this query is returning the expected output but is there any better way to get the expected outcome because using $unwind operator in quite expensive in the sense it may take longer execution time, hoping for positive response.Thanks in advance.

Answer 1

You can use $reduce to replace the second $group stage.
$match
$group - Push variant.price into new array and results nested array of array.
$project:
3.1. $reduce - Use to flatten the nested array from the result 2 by $concat the arrays into one.
3.2. $min - Select min value from the result 3.1.
db.collection.aggregate([
{
$match: {
brand: "lenovo"
}
},
{
$group: {
_id: "$brand",
prices: {
$push: "$variant.price"
}
}
},
{
$project: {
_id: 1,
minPrice: {
$min: {
"$reduce": {
"input": "$prices",
"initialValue": [],
"in": {
"$concatArrays": [
"$$value",
"$$this"
]
}
}
}
}
}
}
])
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