Atlas App Services database trigger doesn't fire for match expression

MongoDB Atlas App Services & Realm
1

Hi,
I’m trying to create a database trigger using the Atlas App Services. I’m having trouble getting my trigger to fire when I use a match expression.

Below is the match expression:

{
    "updateDescription.updatedFields.data.currStreak": {
        "$exists": true
    }
}

So basically I want the trigger to fire for any change in data.currStreak. Please note that currStreak is an integer. This is not working. But if I remove the match expression then I can see the triggers in my logs. I tried printing the changeEvent and below is one such event

{
  "_id": {
    "_data": "8263BE37C6000000042B022C0100296E5A10046DB0EBA4EE3541A0A0DA37F2FA1A1B23463C5F6964003C363339633161333537666161383162633466613961373730000004"
  },
  "operationType": "update",
  "clusterTime": {
    "$timestamp": {
      "t": 1673410502,
      "i": 4
    }
  },
  "ns": {
    "db": "<my_db>",
    "coll": "users"
  },
  "documentKey": {
    "_id": "639c1a357faa81bc4fa9a770"
  },
  "updateDescription": {
    "updatedFields": {
      "data.currStreak": 11
    },
    "removedFields": [
      "_id__baas_transaction"
    ],
    "truncatedArrays": []
  }
}

What can be the issue when the change event is showing the same thing which I wanted with the match expression?

NOTE: I’m on a FREE SHARED CLUSTER. Will that have any effect on this behavior?

Thanks,
Rajeev

2

So I searched and read more, and looks like the issue is the nested field. As can be seen in the above logs, "data.currStreak": 11 it is taking the updated field as a top level field and not as a nested field.

So I have come up with the below match expression which seems to be working. Is it correct? Are there any side effects because of this?

I’ve used $gte as I’m not interested in the change when currStreak becomes 0.

{
  "$expr": {
    "$gte": [
      {
        "$let": {
          "vars": {
            "updated": "$updateDescription.updatedFields"
          },
          "in": {
            "$getField": {
              "field": "data.currStreak",
              "input": "$$updated"
            }
          }
        }
      },
      {
        "$numberInt": "1"
      }
    ]
  }
}

Though I definitely wish and hope that MongoDB can include at least a couple of sentences for nested fields while explaining Database Triggers. And maybe point us in the right direction for creating a good enough match expression.

Thanks,
Rajeev