aggregate with unwind, how to limit per document and not globally? (mongodb) - mongodb

Question

If I have a collection with 300 documents, each document has a array field called items (each item of the array is an object), something like this:
*DOCUMENT 1:*
_id: **********,
title: "test",
desc: "test desc",
items (array)
0: (object)
title: (string)
tags: (array of strings)
1: (object)
etc.
and I need to retrieve items by tags, what I'm using is this query below. I have to $limit results to something like 200 or the query is too big, the problem is if the first document has more than 200 items what it returns are only items of that document, what I'd need is to limit results PER document, for instance I'd need to retrieve 5 items for each different document where tags match ($all) tags provided.
const foundItems = await db.collection('store').aggregate([
{
$unwind: '$items'
},
{
$match: {
'items.tags': { $all : tagsArray }
}
},
{
$project: {
myitem: '$items',
desc: 1,
title: 1
}
},
{
$limit: 200
}
]).toArray()
to make it more clear and simple what I'd need in a ideal world would be something like:
{
$limit: 5,
$per: _id,
$totalLimit: 200
}
instead of $limit: 200 , is this achievable somehow? I didn't find any explanation about it in the official documentation.
What I tried is to add $sort right before $limit which would make sense if it had the behaviour I'm looking for put it that way and maybe not if placed AFTER the limit, but unfortunately it doesn't work that way and placed before or after the limit doesn't make any difference.
And I can't really use $sample since results are more than the 5%

Answer 1

Updated demo - https://mongoplayground.net/p/nM6T9XVa-XK
db.collection.aggregate([
{ $unwind: "$items" },
{
$match: {
"items.tags": {
$all: [ "a","b" ]
}
}
},
{
"$group": {
"_id": "$_id",
"myitem": { "$push": "$items" },
desc: { "$first": "$desc" },
title: { "$first": "$title" }
}
},
{
"$project": {
"_id": 1,
desc: 1,
title: 1,
"myitem": { $slice: [ "$myitem", 2 ]
}
}
},
{
$unwind: "$myitem"
}
])
Demo - https://mongoplayground.net/p/BESptnyUfSS
After matching the records you can $group them according to id and $project them and limit them using Use $slice
db.collection.aggregate([
{ $unwind: "$items" },
{
$match: {
"items.tags": { $all: [ "a", "b" ]
}
}
},
{
$project: {
_id: 1, myitem: "$items", desc: 1,title: 1
}
},
{
"$group": {
"_id": "$_id",
"myitem": { "$push": "$myitem" }
}
},
{
"$project": {
"_id": 1,
"myitem": {
$slice: [ "$myitem", 1 ] // limit records here per group / id
}
}
}
])