mongodb aggregation with filter options - mongodb

Question

I have two collections where I'm trying to do an aggregation query with filter options. I have looked online but I couldn't find solution for this.
Col 1
[
{
_id: ObjectId('st_123'),
stud_num: 123,
school: ObjectId('sc_123'),
gender: 'M'
},
{
_id: ObjectId('st_234'),
stud_num: 123,
school: ObjectId('sc_234'),
gender: 'F'
},
{
_id: ObjectId('st_345'),
stud_num: 123,
school: ObjectId('sc_345'),
gender: 'M'
}
]
Col 2
[
{
_id: ObjectId('f_123'),
stud_health_id: ObjectId('st_123'),
schoolYear: ObjectId('sy123')
},
{
_id: ObjectId('f_234'),
stud_health_id: ObjectId('st_234'),
schoolYear: ObjectId('sy234')
},
{
_id: ObjectId('f_345'),
stud_health_id: ObjectId('st_890'),
schoolYear: ObjectId('sy234')
},
{
_id: ObjectId('f_456'),
stud_health_id: ObjectId('st_345'),
schoolYear: ObjectId('sy345')
}
]
I am trying to filter the records from collection 1 which doesn't have entry in collection 2 with extra params.
If I send {schoolYear: ObjectID('sy234)} then it should return the first and third document of collection 1 because for that year those two students doesn't have record.

Answer 1

One option is using $lookup and $match:
db.col1.aggregate([
{$lookup: {
from: "col2",
as: "col2",
let: {schoolYear: "sy234", stud_id: "$_id"},
pipeline: [
{$match: {$expr: {
$and: [
{$eq: ["$schoolYear", "$$schoolYear"]},
{$eq: ["$stud_health_id", "$$stud_id"]}
]
}
}
}
]
}
},
{$match: {"col2.0": {$exists: false}}},
{$unset: "col2"}
])
See how it works on the playground example