MongoDB Update (Aggregation Pipeline) using values from nested nested Arrays

Question

I'm trying to update a field (totalPrice) in my document(s) based on a value in a nested, nested array (addOns > get matching array from x number of arrays > get int in matching array[always at pos 2]).

Here's an example of a document with 2 arrays nested in addOns:

{
   "_id": ObjectID('.....'),
   "addOns": [
                ["appleId", "Apples", 2],
                ["bananaID", "Bananas", 1]
             ],
   "totalPrice": 5.7
}

Let's say the price of bananas increased by 20cents, so I need to look for all documents that have bananas in its addOns, and then increase the totalPrice of these documents depending on the number of bananas bought. I plan on using a updateMany() query using an aggregation pipeline that should roughly look like the example below. The??? should be the number of bananas bought, but I'm not sure how to go about retrieving that value. So far I've thought of using $unwind, $filter, and $arrayElemAt, but not sure how to use them together. Would appreciate any help!

db.collection('orders').updateMany(
     { $elemMatch: { $elemMatch: { $in: ['bananaID'] } } },
     [
          { $set: {'totalPrice': { $add: ['$totalPrice', { $multiply: [{$toDecimal: '0.2'}, ???] } ] } } }
     ]

I'm not exactly sure whats my mongo version is, but I do know that I can use the aggregation pipeline because I have other updateMany() calls that also use the pipeline without any issues, so it should be (version >= 4.2).

** Edit : Thought of this for the???, the filter step seems to work somewhat as the documents are getting updated, but the value is null instead of the updated price. Not sure why

1. Filter the '$addOns' array to return the matching nested array.
2. For the condition, use $eq to check if the element at [0] ('$$this.0') matches the string 'bananaID', and if it does return this nested array.
3. Get the array returned from step 1 using $arrayElemAt and position [0].
4. Use $arrayElemAt again on the array from step 3 with positon [2] as it is the index of the quantity element

{ $arrayElemAt: [{ $arrayElemAt: [ { $filter: { input: "$addOns", as:'this', cond: { $eq: ['$$this.0', 'bananaID'] } } } , 0 ] }, 2 ] }

Answer 1

Managed to solve it myself - though only use this if you don't mind the risk of updateMany() as stated by Joe in the question's comments. It's very similar to what I originally shared in ** Edit , except you cant use $$this.0 to access elements in the array.

Insert this into where the??? is and it'll work, below this code block is the explanation:

{ $arrayElemAt: [{ $arrayElemAt: [ { $filter: { input: "$addOns", as:'this', cond: { $eq: [{$arrayElemAt:['$$this', 0]}, 'bananaID'] } } } , 0 ] }, 2 ] }

1. Our array looks like this: [["appleId", "Apples", 2], ["bananaID", "Bananas", 1]]
2. Use $filter to return a subset of our array that only contains the elements that matches our condition - in this case we want the array for bananas. $$this represents each element in input, and we check whether the first element of $$this matches 'bananaID'.

{ $filter: { input: "$addOns", as:'this', cond: { $eq: [{$arrayElemAt:['$$this', 0]}, 'bananaID'] } } }

// how the result from $filter should look like
// [["bananaID", "Bananas", 1]]

3. Because the nested banana array will always be the first element, we use $arrayElemAt on the result from step 2 to retrieve the element at position 0.

// How it looks like after step 3: ["bananaID", "Bananas", 1]

4. The last step is to use $arrayElemAt again, except this time we want to retrieve the element at position 2 (quantity of bananas bought)
5. This is how the final updateMany() query looks like after steps 1-4 are evaluated.

db.collection('orders').updateMany(
     { $elemMatch: { $elemMatch: { $in: ['bananaID'] } } },
     [
          { $set: {'totalPrice': { $add: ['$totalPrice', { $multiply: [{$toDecimal: '0.2'}, 1] } ] } } }
     ], *callback function code*)

// notice how the ??? is now replaced by the quantity of bananas which is 1