Update document with aggregation pipeline with $lookup - node.js

Question

I have a product collection which has fields -userId,referenceProductId,
I want to add new field buyerUserId to all doc where its value will be equal to userId for documents where its _id is equal to referenceProduct_id
For example-for following 2 doc
{
"_id": { "$oid": "61ded34c1e7007b17a86f889" },
"userId": { "$oid": "6190b06b113314ad2183db09" },
"referenceProductId": { "$oid": "61ded15fdd1363aa1ce09c55" }
}
{
"_id": { "$oid": "61ded15fdd1363aa1ce09c55" },
"userId": { "$oid": "6190b06b113314ad2183db09" },
"referenceProductId": { "$oid": "61ded34c1e7007b17a86f889" }
}
BuyerUserId for doc1 will be 6190b06b113314ad2183db09 since doc1's _id is equal to referenceProductId of doc2
I am new to mongoDB, trying to update with below code but doesn't work
{ "$match": { "status": "purchased" }},
{ $lookup:{
from:"product",
let:{
"id":"$Id",
"referenceProductId":"$ReferenceProductId",
"userId":"$UserId",
},
pipeline:[
{
$match:{
$expr:{
$eq: ["$$id", "$referenceProductId"] ,
}
}
},
],
as:"products"
}
},{
$project:{
"buyerUserId":"$products.userId"
}
}
])

Answer 1

As noted in a comment above, $merge can be used as "doc to doc" update mechanism. Starting in v4.4 you may output the results of the merge directly back onto the collection being aggregated.
db.foo.aggregate([
{$lookup: {from: "foo",
let: {rid: "$_id"},
pipeline: [
{$match:{$expr:{$eq: ["$$rid", "$referenceProductId"]}}}
],
as: "Z"}}
// If _id->refId match happened, set buyerUserId else do not set
// anything not even null ($$REMOVE means "do nothing"):
,{$project: {buyerUserId: {$cond: [ {$ne:[0,{$size: "$Z"}]}, "$_id", "$$REMOVE"] }} }
// At this point in pipeline we only have _id and maybe buyerUserId. Use _id
// from each doc to merge buyerUserId back into the main collection.
// Since the docs came from this collection, we should fail if something
// does not match.
,{$merge: {
into: "foo",
on: [ "_id" ],
whenMatched: "merge",
whenNotMatched: "fail"
}}
]);