performing $lookup on subset of collection - mongodb

Question

I have this data
[
{"_id":0,"a":1,"b":1,"source":1},
{"_id":1,"a":1,"c":4,"source":1},
{"_id":2,"a":2,"d":6,"source":1},
{"_id":3,"a":2,"e":6,"source":1},
{"_id":4,"a":2,"f":6,"source":1},
{"_id":5,"a":3,"d":6,"source":1},
{"_id":6,"a":3,"b":1,"source":1},
{"_id":7,"a":3,"f":6,"source":1},
{"_id":8,"a":3,"qq":3,"source":2},
{"_id":9,"a":3,"fl":6,"source":2}
]
I want to return all documents whose a field is equal to the a field of a document that has a field b. Furthermore, all must be from source 1.
The final result should be this:
[
{"_id":0,a":1,"b":1,"source":1},
{"_id":1,"a":1,"c":4,"source":1},
{"_id":5,"a":3,"d":6,"source":1},
{"_id":6,"a":3,"b":1,"source":1},
{"_id":7,"a":3,"f":6,"source":1}
]
The following query gives me the results I want:
myCollection.aggregate([{"$match":{"b":{"$exists":true},"source":1}},
{"$group":{"_id":null, "a":{"$addToSet":"$a"}}},
{"$unwind":{"path":"$a"}},
{"$project":{"_id":false}},
{"$lookup":
{"from": "myCollection",
"localField":"a",
"foreignField":"a",
"as":"results"}},
{"$project":{"a":false}},
{"$unwind":{"path":"$results"}},
{"$replaceRoot":{"newRoot":"$results"}},
{"$match":{"source":1}}
])
However, having to add that last {"$match":{"source":1}} statement got me thinking that for large sets of data the $lookup statement is going to produce a lot of unwanted results that will then be filtered out by my last $match statement. Is there any way to prevent their generation by limiting $lookup to documents from myCollection where source equals 1?
ie replace
{"$lookup":
{"from": "myCollection"
with something like
{"$lookup":
{"from": myCollection.match({"source":1})
Alternatively, is there a more efficient pipeline I could be using?

Answer 1

You can filter few documents in the pipeline of $lookup stage. This will help in to gain some performance and avoid unnecessary results. You can use it like below:
{
"$lookup": {
"from": "collection",
"let": {
a_: "$a"
},
"pipeline": [
{
"$match": {
$expr: {
$and: [
{
$eq: [
"$source",
1
]
},
{
$eq: [
"$a",
"$$a_"
]
}
]
}
}
}
],
"as": "results"
}
}
Your $project stage,
{"$project":{"a":false}}
is useless actually, you can omit it.