I have this below collection and I use MongoDB aggregation to acheive an output and trying to merge into another collection.

[
  {
    "order_line_item_id": 1,
    "order_id": 100,
    "products": [
      {
        "name": "Shoe",
        "hasDefect": "YES"
      },
      {
        "name": "Pant",
        "hasDefect": "NO"
      },
      {
        "name": "Shirt",
        "hasDefect": "NOT_SURE"
      }
    ]
  },
  {
    "order_line_item_id": 2,
    "order_id": 100,
    "products": [
      {
        "name": "Shoe",
        "hasDefect": "YES"
      },
      {
        "name": "Pant",
        "hasDefect": "YES"
      },
      {
        "name": "Shirt",
        "hasDefect": "YES"
      }
    ]
  },
  {
    "order_line_item_id": 3,
    "order_id": 101,
    "products": [
      {
        "name": "Shoe",
        "hasDefect": "YES"
      },
      {
        "name": "Pant",
        "hasDefect": "NO"
      },
      {
        "name": "Shirt",
        "hasDefect": "NOT_SURE"
      }
    ]
  }
]

The output can be a new document altogether or an update for one of the products in the order_line_item document (like below).

[
  {    
    "order_id": 100,
    "products": [
      {
        "name": "Shoe",
        "hasDefect": "YES"
      },
    ]
  }
]   

I m using the merge aggregation stage to insert/update to another mongoDB collection.
{ $merge: { into: "order", on: "order_id", whenMatched: "replace", whenNotMatched: "insert" }}

The issue is it is an update operation and the order has previous products it replace all existing product with this new one.
For example if the order has the below document, it should update only “Shoe” product.

[
  {    
    "order_id": 100,
    "products": [
      {
        "name": "Shoe",
        "hasDefect": "NO"
      },
      {
        "name": "Pant",
        "hasDefect": "NO"
      },
      {
        "name": "Shirt",
        "hasDefect": "NOT_SURE"
      }
    ]
  }
]