How to remove _id from aggregate $match & $group query - node.js

Question

I am trying to display unique records grouped by the particular slug passed in.
My output in postman looks like this though:
"subcats": [
{
"_id": {
"subcategory": {
"_id": "5d2b42c47b454712f4db7c37",
"name": "shirts"
}
}
}
]
when my desired output is:
"subcats": [
{
"_id": "5d2b42c47b454712f4db7c37",
"name": "shirts"
}
]
An example of a product in the database:
"_id": "5d39eff7a48e6e30ace831dc",
"name": "A colourful shirt",
"description": "A nice colourful t-shirt",
"category": {
"_id": "5d35faa67b19e32ab3dc91ec",
"name": "clothing",
"catSlug": "clothing"
},
"subcategory": {
"_id": "5d2b42c47b454712f4db7c37",
"name": "shirts",
"catSlug": "shirts"
},
"price": 19
}
I don't want that top level _id there with everything nested inside of it.
I tried using $project but then I just end up with an empty array.
const products = await Product.find({ "category.catSlug": catslug }).select({
name: 1,
description: 1,
price: 1,
category: 1
});
const subcats = await Product.aggregate([
{ $match: { "category.catSlug": catslug } },
{ $group: { _id: { subcategory: "$subcategory" } } }
{ $project: { _id: 0, name: 1 } }
]);
Promise.all([products, subcats]);
res.status(200).json({
products,
subcats
});

Answer 1

What I have understood from your question is you need product by category and all sub categories as a list.
Below is the aggregation:
db.getCollection('test').aggregate([
{ $match: { "category.catSlug": "clothing" } },
{ $group: { _id: '$subcategory.name', subCategory: { $first : "$subcategory" }}},
{ $group: { _id: null, subcats: { $push: {_id: "$subCategory._id", name: "$subCategory.name" }}}},
{ $project: {_id: 0, subcats: 1}}
]);
Note: You can directly push "$subcategory" but it will have catSlug inside object
Output:
{
"_id" : null,
"subcats" : [{
"_id" : "5d2b42c47b454712f4db7c37",
"name" : "shirts"
},
{
"_id" : "5d2b42c47b454712f4db7c37",
"name" : "pents"
}]
}
Hope this help!

Answer 2

Hi you need to assign the required data in other fields while projection then define the _id to 0 as given below
{$project:{subcat_data:'$_id.subcategory'},_id:0}