LiveQuery fields does not work as expected

[dblythy]

New Issue Checklist

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

Issue Description

According to the LiveQuery protocol, the fields parameter can be passed to only listen to updates on certain fields. However, this functions more like "select", where the payload only includes those keys, even if they haven't been updated.

Suppose the ParseObject Player contains three fields name, id and age. If you are only interested in the change of the name field, you can set query.fields to [name]

Steps to reproduce

Create a subscription using the fields parameter, update any other key.

Actual Outcome

Live query is triggered

Expected Outcome

Live query should only trigger if the fields are updated (or new).

Here is an example of a failing test. No matter how many times an object save is called, the spy should only be called if the fields are selected.

it('can handle live query with fields', async () => {
    await reconfigureServer({
      liveQuery: {
        classNames: ['Test'],
      },
      startLiveQueryServer: true,
    });
    const query = new Parse.Query('Test');
    query.select('yolo');
    const subscription = await query.subscribe();
    const spy = {
      create(obj) {
        if (!obj.get('yolo')) {
          fail('create should not have been called')
        }
      },
      update(object, original) {
        if (object.get('yolo') === original.get('yolo')) {
          fail('create should not have been called')
        }
      }
    }
    const createSpy = spyOn(spy, 'create').and.callThrough()
    const updateSpy = spyOn(spy, 'update').and.callThrough()
    subscription.on('create', spy.create);
    subscription.on('update', spy.update);
    const obj = new Parse.Object('Test');
    obj.set('foo', 'bar');
    await obj.save();
    obj.set('foo', 'xyz');
    obj.set('yolo', 'xyz');
    await obj.save();
    const obj2 = new Parse.Object('Test');
    obj2.set('foo', 'bar');
    obj2.set('yolo', 'bar');
    await obj2.save();
    obj2.set('foo','bart');
    await obj2.save();
    await new Promise(resolve => setTimeout(resolve, 2000));
    expect(createSpy).toHaveBeenCalledTimes(1);
    expect(updateSpy).toHaveBeenCalledTimes(1);
  })

Environment

Server

• Parse Server version: 5.3
• Operating system: macos
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): local

Database

• System (MongoDB or Postgres): mongo
• Database version: 5.1
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): local

Client

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

Logs

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[mtrezza]

Suppose the ParseObject Player contains three fields name, id and age. If you are only interested in the change of the name field, you can set query.fields to [name]

I think the common interpretation is that LiveQuery should only trigger for the fields set, but this paragraph can be ambiguous, it's missing a clear description of the functionality. Does it more explicitly mention that LiveQuery only triggers for these fields when fields is set?

The parameter name is also not well chosen - "fields" does not imply any functionality.

If we want to avoid making this a breaking change, we may use this opportunity to deprecate the fields param and add a better named param triggerFields (or similar) with the new behavior.

[dblythy]

The full paragraph is here https://github.com/parse-community/parse-server/wiki/Parse-LiveQuery-Protocol-Specification#subscribe-message

I assume that the functionality of "if you are only interested in the change of the name field" is different to the existing functionality of select. I think creating a param triggerFields and updating the spec accordingly is a good suggestion. Perhaps in the future we could also rename the param fields to select

[dblythy]

Actually what do you think of:

query.whereChanged(...keys) query.changed(...) query.triggerFields(...) query.listen(...) query.restrict(...) query.selected(...) query.alter(...)

[mtrezza]

I changed this from bug to feature, since the PR also seems to be a feature PR, if I got this right?

Perhaps in the future we could also rename the param fields to select

That's a clever suggestion; but if fields currently behaves like select then why do we maintain fields at all? If the docs currently imply that fields behaves like a combination of select and triggerFields, then this PR doesn't correct this bug / ambiguity if we leave fields as is. On the other hand we also don't want an unnecessary breaking change to fix this.

Maybe a clean approach to address the whole issue in a single PR:

• deprecate fields with a changelog comment: "docs are ambiguous, so we phase this out; use select and/or the new triggerFields to achieve desired behavior"
• add triggerFields

[mtrezza]

query.whereChanged(...keys) query.changed(...) query.triggerFields(...) query.listen(...) query.restrict(...) query.selected(...) query.alter(...)

You mean as method name instead of triggerFields? Yes, triggerFields sounds unnecessarily complex (2 words) - a single word would be better, and it should be a verb instead of a noun. listen or watch sounds intuitive to me.

[cbaker6]

I don’t know why it wasn’t just named select. Perhaps in a past version it did behave differently, but the feature somehow got overriden.

It looks like the fields isn't implemented in LiveQuery for the Objective-C and Android SDK's when I looked through the repos. This is also confirmed in https://github.com/parse-community/ParseLiveQuery-Android/pull/18. I wonder if the released spec was slightly different than the one that was used internally, or if it was just implemented slightly differently after it was made.

[dblythy]

@cbaker6 I'm thinking that too. Perhaps the spec was built before the feature was built, and it ended up behaving the same as select. So I suppose this issue is just to update the spec, and then introduce the new feature listen.

[cbaker6]

I believe this comment also supports what you mentioned about fields: https://github.com/parse-community/Parse-SDK-JS/pull/518#issue-278341162

[dblythy]

@mtrezza could you please update the spec:

https://github.com/parse-community/parse-server/wiki/Parse-LiveQuery-Protocol-Specification#subscribe-message

[mtrezza]

Sure, do you have a suggestion I can copy/paste?

Interesting this is a GH wiki. Do we need to update anything in the docs as well?