mongo - get count of a intermediary document - mongodb

Question

currently I have a query:
const result = await getInstances();
that provides me an array of document:
[{name: "first", age: 13},
{name: "second", age: 21},
{name: "third", age: 11},
{name: "fourth", age: 14}
...]
The query goes something like this:
...
return Instances.aggregate
.match({//condition})
.skip(skipValue).limit(pageSize) // pagination done here
I want a query that appends a count for the total no. of documents before the pagination, but returns the paginated data, e.g:
...
return Instances.aggregate
.match({//condition}) ## I WANT THE COUNT OF THIS STEP TO BE APPENDED
.<SOME_PIPELINE_HERE>
.skip(skipValue).limit(pageSize) // pagination done here
would return something like:
{
data: [{name: "first", age: 12}....<ALL_PAGINATED_DATA>],
totalCount: 54 #count of data before pagination
}
What I tried and didn't work:
Instances.aggregate()
.match({//CONDITION})
.addFields({count: {$size: _id}})
.skip(value).limit(value)
It seems it goes through and calculates this for each document instead of the whole

Answer 1

One option is to use $facet in order to "fork" the query in the middle, so the same data can be used on different pipelines. For example:
db.collection.aggregate([
{$match: {a: {$in: [7, 8, 9]}}},
{
$facet: {
total: [{$group: {_id: null, count: {$sum: 1}}}],
data: [{$skip: 1}]
}
},
{$project: {data: 1, total: {$first: "$total.count"}}}
])
See how it works on the playground example