MongoDb return records only if $lookup cond is occur - mongodb

Question

I have a class model which has field ref.
I'm trying to fetch only records that match the condition in lookup.
so what i did:
{
$lookup: {
from: 'fields',
localField: "field",
foreignField: "_id",
as: 'FieldCollege',
},
},
{
$addFields: {
"FieldCollege": {
$arrayElemAt: [
{
$filter: {
input: "$FieldCollege",
as: "field",
cond: {
$eq: ["$$field.level", req.query.level]
}
}
}, 0
]
}
}
},
The above code works fine and returning the FieldCollege if the cond is matched.
but the thing is, i wanted to return the class records only if the FieldCollege is not empty.
I'm totally new to mongodb. so i tried something like this:
{
$match: {
'FieldCollege': { $exists: true, $ne: [] }
}
},
Obv this didn't work.
does mongodb support something like this or am i complicating things?
EDIT:
the result from the above code:
"Classes": [
{
"_id": "613245664c6ea614e001fcef",
"name": "test",
"language": "en",
"year_cost": "3232323",
"FieldCollege":[] // with $unwind
}
],
expected Result:
"Classes": [
// FieldCollege is empty
],

Answer 1

I think the good option is to use lookup with pipeline, and see the final version of your query,
$lookup with fields collection and match your both conditions
$limit to result one document
$match FieldCollege is not empty []
$addElemAt to get first element from result FieldCollege
[
{
$lookup: {
from: "fields",
let: { field: "$field" },
pipeline: [
{
$match: {
$and: [
{ $expr: { $eq: ["$$field", "$_id"] } },
{ level: req.query.level }
]
}
},
{ $limit: 1 }
],
as: "FieldCollege"
}
},
{ $match: { FieldCollege: { $ne: [] } } },
{
$addFields: {
FieldCollege: { $arrayElemAt: ["$FieldCollege", 0] }
}
}
]