Keys are stripped upon subscription update with LiveQuery.select

[dblythy]

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 versions of Parse Server and the Parse JS SDK.

Issue Description

When you have a LiveQuery using .select, the Parse JS SDK removes all other data from any existing matching object.

Steps to reproduce

1. Create an object using .save, such as:

const obj = new Parse.Object('Test')
obj.set('foo', 'bar');
await obj.save();

2. Use query.subscribe on that object, using .select for certain keys

const subscription = await new Parse.Query('Test').select('key').subscribe();

4. Trigger a subscription event from the cloud / a different client
5. Now obj.get('foo') will be undefined

Actual Outcome

Incoming Live Query payload overrides all keys

Expected Outcome

New incoming changes to merge with existing object

Environment

Server

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

Database

• System (MongoDB or Postgres): mongo
• Database version: 5
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): atlas

Client

• Parse JS SDK version: 3.4.2

Logs

[parse-github-assistant]

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[dblythy]

It seems that if an update event is triggered, Parse.Object.fromJSON with override is called:

Parse-SDK-JS/src/LiveQueryClient.js

Line 422 in 8115e95

const parseObject = ParseObject.fromJSON(data.object, override);

Which creates a new Parse.Object with the new keys, and then calls object.clearServerData:

Parse-SDK-JS/src/ParseObject.js

Line 1856 in 8115e95

o._clearServerData();

As serverData is the original object, all these keys are set to undefined, and the object is updated according to the LQ payload (which only has selected fields).

Parse-SDK-JS/src/ParseObject.js

Line 233 in 8115e95

unset[attr] = undefined;

[mtrezza]

So in your example, the LiveQuery event would only contain the key property (if it was defined)?

[dblythy]

In the example, the live query event will only contain the “key” property, so it will unset all other keys on the object

[mtrezza]

What is the expected behavior of a select condition in a LiveQuery?

[dblythy]

The object returned by the live query should only contain the selected keys, but it shouldn’t override other objects with the same IDs that have previously fetched keys

[dblythy]

In my setup I have a live query that applies to one column of an object. All other keys are fetched with the initial query.

whenever the livequery is triggered with that singular key selected, all keys on the original object get set to undefined, as the object has the same state reference as the live query object.

const obj = await new Parse.Query("Test").first()
// obj.get("name") is defined
// objects keys are key, name, cash, year 
const subscription = await new Parse.Query("Test").select("key").subscribe()
subscription.on("update", (object) => {
  // if object.id === obj.id, Parse JS uses the incoming object’s state and assigns it to obj. 
  // obj.get("name") is now undefined as a side effect to the live query, even though no additional code has been written 
 // objects keys are key, and that’s it (as expected)
// obj keys are key (not expected)
});

[mtrezza]

Got it, thanks for the detailed example. Yes, this looks like a bug.

[cbaker6]

I suspect there is a reason for this as LQ uses fields to achieve similar behavior to “select”.

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”. In this situation, when the change of a Player ParseObject fulfills the subscription, only the name field will be sent to the clients instead of the full Player ParseObject. If this is called multiple times, then all of the keys specified in each of the calls will be received.

This is also noted in the spec: https://github.com/parse-community/parse-server/wiki/Parse-LiveQuery-Protocol-Specification#subscribe-message

[dblythy]

Yes, that's not the issue, it's that it overrides all other keys in objects in memory with the same internal reference, meaning that if you first query the three fields name, id and age, and then an LQ event only contains "age", the original queried object in memory will only contain the key age now

Also I don't think the fields parameter even works as expected. See parse-community/parse-server#8027

[mtrezza]

@cbaker6 Did #1487 (comment) answer your concern? If not, let's discuss, if yes I'd go ahead and merge the PR.