slice in array not working? - node.js

Question

This is my mongoose collection data:
{
"ShopId" : "439",
"productName" : "veg",
"productCategory" : "meals",
"mrp" : "38 "
},
{
"ShopId" : "439",
"productName" : "non-veg",
"productCategory" : "meals",
"mrp" : "380 "
},{....}
Query
db.getCollection('ProductDetails').aggregate(
[{ "$match": { "ShopId": "439" } },{"$group": {"_id": "$productCategory", "count": { "$sum": 1 },
"products": {"$push":{"productname": "$productName"}}}},
{"$group": {"_id": null, "productList": {"$push": {"categoryname": "$_id", "productcount": "$count",
"products": "$products"}}}},{$project:{products:{$slice:["$productList.products",2]}}}])
Output:
{
"_id" : null,
"productList" : [
{
"categoryname" : "meals",
"productcount" : 8.0,
"products" : [
{
"productname" : "non veg"
},
{
"productname" : "veg"
},
{
"productname" : "c"
},
{
"productname" : "d"
},
{
"productname" : "df"
},
{
"productname" : "dfr"
},
{
"productname" : "qe"
},
{
"productname" : "as"
}
]
}
]
}
expected output:
I want to limit the number of products to 2.But instead of that all products are displaying.
{
"_id" : null,
"productList" : [
{
"categoryname" : "meals",
"productcount" : 8.0,
"products" : [
{
"productname" : "non veg"
},
{
"productname" : "veg"
}
]
}
]
}

Answer 1

Replace your $project stage with below.
{$project:{products:{$slice:[{$arrayElemAt:["$productList.products", 0]},2]}}}
Your products is array of arrays.
"products": [
[{
"productname": "veg"
}, {
"productname": "non-veg"
}]
]
$arrayElemAt with 0 will pick the inner array and you can use $slice to limit the products.

Answer 2

I believe you are using the $slice function wrong: As I mentioned in this post:
Find a value in array
The slice function takes two parameters: The first is the initial index and the second is the number of elements after this index. Here's an example:
db.collection.find({},{_id:0,products:{$slice : [0,2]})
This will take two elements from the index [0] of the array. Hope my answer was helpful.