Mongodb update pipeline update a field of a element inside a nested array

I have a collection that contains objects such as this.

{
  materials: {
    "m1": {
      inventory: [
        {
          price: 100,
          amount: 65
        }
      ]
    }
  }
}

As you can see inventory is an array deep inside the hierarchy. I want to update the amount field of it.

From the client I receive the material id ("m1") and the inventory index (0).

I have to use an update pipeline because I am setting and unsetting some other fields in this document.

This is what I tried:

await products.findOneAndUpdate(filters, [
  {
    $set: {
      "materials.m1.inventory.0.amount": 100,
    },
  },
]);

But it creates a new field named 0 inside the 0th element and sets the amount inside that object. So the resulting document looks like this.

{
  materials: {
    "m1": {
      inventory: [
        {
          0: {
            amount: 100
          }
          price: 100,
          amount: 65
        }
      ]
    }
  }
}

Whereas what I want is this:

{
  materials: {
    "m1": {
      inventory: [
        {
          price: 100,
          amount: 100
        }
      ]
    }
  }
}

The only way for me to identify which element in the array to update is the index of it.

I am using nodejs mongodb driver. How to write the $set stage for this update pipeline?

I don't think any other straightway to update the specific index position element using aggregation pipeline,

• $range to generate array from 0 to length of materials.m1.inventory array
• $map to iterate loop of above range array
• $cond to check current element is 0 then go to update part otherwise return element from materials.m1.inventory by input index
• $arrayElemAt to get specific element from materials.m1.inventory
• $mergeObjects to merge current object with amount updated property

await products.findOneAndUpdate(filters, [
  {
    $set: {
      "materials.m1.inventory": {
        $map: {
          input: {
            $range: [0, { $size: "$materials.m1.inventory" }]
          },
          in: {
            $cond: [
              { $eq: ["$$this", 0] }, // 0 position
              {
                $mergeObjects: [
                  { $arrayElemAt: ["$materials.m1.inventory", "$$this"] },
                  { amount: 100 } // input amount
                ]
              },
              { $arrayElemAt: ["$materials.m1.inventory", "$$this"] }
            ]
          }
        }
      }
    }
  }
]);

Playground