Update last element of a nested array in mongodb

Question

I have tried the below query of mongoose which does not seem to work:

Model.findOneAndUpdate(
    {
        name: origin
    },
    {
        $set: {
            'field1.$[id1].field2.-1': "value"
        }
    },
    {
        arrayFilters: [
            { 'id1.userId': "customerId" }
        ],
        new: true
    }
);

Note: field1 and field2 are arrays

The negative indexes are not accepted by MongoDB which is causing problems.

mickl · Accepted Answer · 2021-12-21 13:30:02Z

1

You may consider using the $set (aggregation) operator and use double $map operator:

db.collection.aggregate([
    { $match: { name: "myname" } }
    {
        $set: {
            field1: {
                $map: {
                    input: "$field1",
                    in: {
                        $cond: {
                            if: { $ne: [ "$$this.userId", "customerId" ] },
                            then: "$$this",
                            else: {
                                $mergeObjects: [
                                    "$$this",
                                    { 
                                        field2: { 
                                            $concatArrays: [
                                                { $slice: [ "$$this.field2", { $add: [ { $size: "$$this.field2" }, -1 ] } ] },
                                                ["value"]
                                            ]
                                        } 
                                    }
                                ]
                            }
                        }
                    }
                }
            }
        }
    }
])

Mongo Playground

answered Dec 21, 2021 at 13:30

mickl

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5 Comments

ABHISHEK Joshi Over a year ago

This solution does the job. However, the issue here is that this will map through every element even after the required element is updated. Is there a way to optimize this query further? (There will only be 1 element which will satisfy the condition)

mickl Over a year ago

@ABHISHEKJoshi no other single round-trip solution comes to my mind, you can wait for others to respond. Since negative indexes are not allowed you need to know the index of your last element and this requires to query the document first. In my solution everything is processed on the database side

ABHISHEK Joshi Over a year ago

I'm getting this error: MongoServerError: Updating the path 'analysis' would create a conflict at 'analysis'. Please note that I'm using findOneAndUpdate() and not aggregate().

mickl Over a year ago

@ABHISHEKJoshi I think it requires aggregate

ABHISHEK Joshi Over a year ago

Actually I wanted updated document to be returned. That's the reason I used findOneAndUpdate. Maybe I'll have to query the DB twice. I guess that's the only way to go.

maxagno3 · Accepted Answer · 2021-12-21 15:25:14Z

0

Apply the $set operator together with the $ positional operator in your update to change the name field.

The $ positional operator will identify the correct element in the array to update without explicitly specifying the position of the element in the array, thus your final update statement should look like:

db.collection.update(
    { "friends.u.username": "michael" }, 
    { "$set": { "friends.$.u.name": "hello" } }
)

Answer taken from - https://stackoverflow.com/a/34431571

answered Dec 21, 2021 at 15:25

maxagno3

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1 Comment

ABHISHEK Joshi Over a year ago

As mentioned in the title, I want to update last element of nested array. Your solution updates any element which satisfies the condition in 1st argument of the update function and not specifically the last element. So this cannot be a valid solution.

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