mongodb $lookup with findOne mode - mongodb

Question

I want to join two mongodb collections, collectionA and collectionB.
For each document in collectionA I want to check if exists a coincidence in collectionB.
If I do it in a $lookup, it returns all the documents joined, but I would like the search in collectionB stops as soon as one coincidence ​is found (kind of a mongodb findOne). My concern is the performance, I know I could get just the element 0 from the array.
Is there a way to do it using the mongodB aggregation framework?
Example:
collectionA:
[
{
"_id": 1,
"item": "almonds"
},
{
"_id": 2,
"item": "pecans"
}
]
colectionB:
[
{
"_fid": 1,
"date": "2021-01-10"
},
{
"_fid": 1,
"date": "2021-01-11"
},
{
"_fid": 1,
"date": "2021-01-12"
},
{
"_fid": 2,
"date": "2021-01-03"
}
]
$lookup mongoDb
db.colectionA.aggregate([
{
"$lookup": {
"from": "colectionB",
"localField": "_id",
"foreignField": "_fid",
"as": "matches"
}
}
])
Result
[
{
"_id": 1,
"item": "almonds",
"matches": [
/* I don't want this array, with 1 element would be enough */
{
"_fid": 1,
"_id": ObjectId("5a934e000102030405000002"),
"date": "2021-01-10"
},
{
"_fid": 1,
"_id": ObjectId("5a934e000102030405000003"),
"date": "2021-01-11"
},
{
"_fid": 1,
"_id": ObjectId("5a934e000102030405000004"),
"date": "2021-01-12"
}
]
},
{
"_id": 2,
"item": "pecans",
"matches": [
{
"_fid": 2,
"_id": ObjectId("5a934e000102030405000005"),
"date": "2021-01-03"
}
]
}
]
You can test on this mongo playground.
Thanks in advance

Answer 1

If you're using at least MongoDB 3.6, you can execute an aggregation pipeline on a joined collection. It might look like this:
db.colectionA.aggregate([
{
"$lookup": {
"from": "colectionB",
"as": "matches",
"let": {
"fid": "$_id"
},
"pipeline": [
{
"$match": {
"$expr": {
"$eq": [
"$_fid",
"$$fid"
]
}
}
},
{
"$limit": 1
}
]
}
}
])
Working Mongo playground