https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/

Whole document is written to the "inventory_docs" field. Is there any way to shrink or $project the document which is going to written to "inventory_docs"

db.orders.aggregate([{
  $lookup: {
    from: "inventory",
    localField: "item",
    foreignField: "sku",
    as: "inventory_docs"
  }
}])

What I am saying is, rather than this;

{
  "_id": 1,
   "item": "abc",
  "price": 12,
  "quantity": 2,
  "inventory_docs": [
    { "_id": 1, "sku": "abc", description: "product 1", "instock": 120 }
  ]
}

I want to create it as;

{
  "_id": 1,
  "item": "abc",
  "price": 12,
  "quantity": 2,
  "inventory_docs": [
    { "_id": 1, "instock": 120 }
  ]
}

How can I do it?