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

user_1150725 0
Viewed 622

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?

mongodb-nodejs-driver aggregation-framework mongodb
2 Answers

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

user_89871280

The problem with findOneAndUpdate using an aggregation pipeline is that you can only use $set, $unset and $replaceWith (with their aliases). It seems like that $set, or $addFields, is not capable of overwriting objects inside arrays, so you can simply re-create the whole array with the changes you need. You are basically forced to build your query like this:

db.collection.update(filters,
[
  {
    "$addFields": {
      "materials.m1.inventory": {
      ...
      }
    }
  }
])

This is my implementation but there can be more straightforward ways (See edit that replaces the single-object array):

db.collection.update(filters,
[
  {
    "$addFields": {
      "materials.m1.inventory": {
        $concatArrays: [
          [
            {
              price: {
                $getField: {
                  field: "price",
                  input: {
                    $first: "$materials.m1.inventory"
                  }
                }
              },
              amount: 100
            }
          ],
          {
            $slice: [
              "$materials.m1.inventory",
              1,
              {
                $size: "$materials.m1.inventory"
              }
            ]
          }
        ]
      }
    }
  }
])

I am using $concatArrays to concat:

  1. An array with a single object, which is the one at index 0 but with the amount field set to 100
  2. The slice of the inventory array starting from index 1.

Obviously, if the index was different than 0, you would have to take a first slice inventory[0:index], a single-object array (see point 1), and the slice inventory[index + 1:].

Here the demo.

EDIT: The $mergeObjects part proposed by @turivishal is definitely better than my single-object array: whether you have 2 or 20 fields the code is the same.

user_55873930
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