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

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: [...]
}]

question from:https://stackoverflow.com/questions/65642298/combining-2-collections-after-filtering-2nd-one-based-on-ids-from-first-one

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