Combining 2 collections after filtering 2nd one based on ids from first one - mongodb

Question

I have 2 collections like this:
Batches:
{
"userId": "",
"productId: "",
}
Products:
{
"_id": "",
"name": "",
}
What i want to do is filter the batches by userId first. And then get all products which i get ids in filtered elements of batches. I have seen a lot of examples but mostly goes the opposite way.
My final result i would like to look like this:
[{
name: "product 1",
batches: [...]
}]

Answer 1

You can try lookup with aggregation pipeline,
$lookup in branches collection, pass productId in let, and match expression condition for for productId and filter userId
db.products.aggregate([
{
$lookup: {
from: "branches",
let: { productId: "$_id" },
pipeline: [
{
$match: {
userId: "1", // filter user id here
$expr: { $eq: ["$$productId", "$productId"] }
}
}
],
as: "brnaches"
}
}
])
Playground

Answer 2

Maybe something like this:
mongos> db.Batches.find()
{ "_id" : ObjectId("5ff9a66fda146da4e1359dc9"), "userId" : "User1", "productId" : "product1" }
{ "_id" : ObjectId("5ff9a675da146da4e1359dca"), "userId" : "User2", "productId" : "product1" }
{ "_id" : ObjectId("5ff9a682da146da4e1359dcb"), "userId" : "User3", "productId" : "product2" }
{ "_id" : ObjectId("5ff9a6deda146da4e1359dcc"), "userId" : "User3", "productId" : "product1" }
mongos> db.Products.find()
{ "_id" : "product1", "name" : "product 1" }
{ "_id" : "product2", "name" : "product 2" }
{ "_id" : "product3", "name" : "product 3" }
{ "_id" : "product4", "name" : "product 4" }
mongos> db.Batches.aggregate([ {$match:{"userId":{$in:["User3","User1"]}}}, { $lookup:{ from:"Products" , localField:"productId" , foreignField:"_id" , as:"match" } } , {$unwind:"$match"} , {$project:{ name:"$match.name" , batches :{userId:"$userId" , productId:"$productId"} }} , {$group:{_id:"$name" , batches:{$addToSet:"$batches"}}} , {$project:{_id:0, name:"$_id" , batches:1}} ])
{ "batches" : [ { "userId" : "User1", "productId" : "product1" }, { "userId" : "User3", "productId" : "product1" } ], "name" : "product 1" }
{ "batches" : [ { "userId" : "User3", "productId" : "product2" } ], "name" : "product 2" }
mongos>