Using cond in mongo aggregate match expression - node.js

Question

So i'm trying to query products based on the user specified filter. if the user specifies the size, for example XS, then i fetch all the products with 'XS' sizes, otherwise if the size isn't specified then i will fetch all the products without checking for their size. How can i do it with the mongo aggregation ?
here's my code.
1st: here's 2 products documents from my collection
[
{
"_id": "6124d16b1396d20aea617d75",
"username": "josh988",
"userpic": "josh.jpg",
"adresse": "545 Strawberry Lane Mokena, IL 60448",
"city": "Paris",
"country": "France",
"product": {
"_id": "62c064103a7b5160e957764c",
"images": [
"img1734116er1111111.jpeg"
],
"name": "White shirt",
"category": "Kids",
"subCategory": "Tshirts",
"size": "XS",
"price": 150
}
},
{
"_id": "6124d16b1396d20aea617d75",
"username": "laurel22",
"userpic": "laurel.jpg",
"adresse": "545 Strawberry Lane Mokena, IL 60448",
"city": "Lyon",
"country": "France",
"product": {
"_id": "62c064103a7b5160e957764c",
"images": [
"img1711656775696063.jpeg"
],
"name": "Black shirt",
"category": "Kids",
"subCategory": "Tshirts",
"size": "M",
"price": 250
}
},
]
to make the matter simple , my body api body request looks like this
{
"selectedCategory": "Kids",
"selectedSubCategory": "Tshirts",
// for e.g selectedSize could be XS
"selectedSize": null
}
So what i'd like to do is, if the user select a size then fetch the products with it otherwise fetch everything.
I tried this with mongo but didn't work
const searchResults = await user.aggregate([
{
$match: {
"product.category": `${req.body.selectedCategory}`,
"product.subCategory": `${req.body.selectedSubCategory}`,
$expr: {
$cond: {
// what should i do here ? or is there another way of achieving this
if: { $ne: [`${req.body.selectedSize}`, null] },
then: { product.size : req.body.selectedSize },
// and what should write here to fetch everything
else: { product.size: "" },
},
},
},
},
]);

Answer 1

You can generate the aggregation pipeline conditionally in nodejs itself. Like this:
let matchStage = {
$match: {
"product.category": `${req.body.selectedCategory}`,
"product.subCategory": `${req.body.selectedSubCategory}`,
}
};
if(req.body.selectedSize) {
matchStage = {
$match: {
...matchStage["$match"],
"product.size": request.body.selectedSize
}
};
}
const searchResults = await user.aggregate([matchStage]);