How do I get how many exams have been done - node.js

Question

I have 3 collections: Categories, Exams and Results.
For each Category I have some exams. If one exam is done, I add its result on "Result" collection.
I have take all exams for each category.
The problem is that I can't count how many of them are done.
This is what I get right now:
{ category: "Test", total_exams: 10 }
This is what I want:
{ category: "Test", total_exams: 10, done_exam: 4 }
Below you can see what I have done until now:
https://mongoplayground.net/p/upSAPkYe4DR

Answer 1

Quick fixes:
second $lookup stage, let to pass examsCount._id,
inside pipeline $match stage match expression, $exam_id with $$examId
put user_id match condition outside expression
$count total documents in count
$addFieldsto getdoneExams.countthis will return count in zero index, get it using$firstor$arrayElemAt`
db.categories.aggregate([
{
$lookup: {
from: "exams",
localField: "_id",
foreignField: "categoryId",
as: "examsCount"
}
},
{
$lookup: {
from: "results",
let: { examId: "$examsCount._id" },
pipeline: [
{
$match: {
$expr: {
$and: [{ $in: ["$exam_id", "$$examId"] }]
},
user_id: 22222222
}
},
{ $count: "count" }
],
as: "doneExams"
}
},
{
$addFields: {
examsCount: { $size: "$examsCount" },
doneExams: { $first: "$doneExams.count" }
}
}
])
Playground
There are many approaches to get this result, i have just resolved your issue in your approach.