Realm Trigger match on nested array update

MongoDB Atlas Atlas Triggers & Functions
8

When it comes to arrays, the challenge is the index part (i ) of the field name "matrix.items.i.field" , which dynamically represents what element in the array has been updated.

So, the rule to match those events would involve a regular expression, matching all "matrix.items.i.field no matter the value of i or field:

{
    "$expr": {
        "$gt": [
            {
                "$size": {
                    "$filter": {
                        "input": { "$objectToArray": "$updateDescription.updatedFields" },
                        "cond": { "$regexMatch": { "input": "$$this.k", "regex": "^matrix\\.items\\..*$" } }
                    }
                }
            },
            { "$numberInt": "0" }
        ]
    }
}

By tweaking the regex part of this rule, it can be applied to any other use case where a trigger needs to fire from an update to an array element.

For example, if the trigger needs to fire only when the pos field updates, ignoring updates to context_id or date_added fields, changing regex to "^matrix\\.items\\..*\\.pos$" would provide that functionality.

The rest of the event handling logic must then be implemented in the corresponding Function.

Best,
Alex

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