'array' type column is updated even without an explicit 'set'

[refre5h]

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

It seems that Parse recognizes changes in the 'array' column and will save the changes even without an explicit 'set'.

Steps to reproduce

const files = object.get('files'); // files is an array
files[0].aa = 'bb';
console.log(object.dirtyKeys()); // ==> ['files']

object.set('anotherCol', someVal);
object.save(); // the object's 'anotherCol' and 'files' columns are both updated.

Actual Outcome

'files' is a dirty key and is updated upon .save() even without an explicit 'set'.

Expected Outcome

'files' is not a dirty key unless user 'set' the change like: object.set('files', value)

Environment

Server

• Parse Server version: 5.2.0
• Operating system: Amazon Linux 2
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): AWS

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 5.0.17
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): MongoDB Atlas,

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): JavaScript
• SDK version: 3.4.0

Logs

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[mtrezza]

Classified as bug as this seem unexpected behavior.

[dblythy]

There are some JS SDK tests to make sure this behaviour happens

[mtrezza]

This behavior reminds me of the "dot notation" feature of the Parse JS SDK. In that context I would expect it. Outside of that I personally would not because it's missing an explicit set, even thought it seems convenient.

Is this just an alternative way to make an array element dirty, or is it the only way? If it's the only way then it may be a deeper design choice for lack of a better syntax. If it's an alternative to using set explicitly like object.set('files[0]', 'bb'); (if that even works) then it may be considered an unexpected convenience feature.

[dblythy]

I think the purpose of this behaviour is for relations, as you can call:

const obj = new Parse.Object("Test");
const relation = obj.relation("key");
relation.add(obj);
// no need to call obj.set

https://github.com/parse-community/Parse-SDK-JS/blob/0fd7141808f3ad51e7200b3e4bf25093b2c5c354/src/__tests__/ParseObject-test.js#L828

[mtrezza]

Then maybe this is just a docs issue and we need to document this behavior. Is it possible to set an array element any other way, using set like object.set('files[0]', 'bb');?