Mongo explain results incorrectly returning an object instead of an array of objects

[cbaker6]

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

Mongo explain results incorrectly returning an object instead of an array of objects. This causes an issue for SDKs that use strongly typed languages as results (plural) should always be an array.

This will break any SDK that isn't strongly typed. In the case of the JS SDK, it will break current testcases instead (PR for JS SDK here. In addition, this brings up another issue in the JS SDK which doesn't support explain for first queries.

Originally discussed on community.parseplatform.org

Steps to reproduce

Run any explain query.

Actual Outcome

A single explain object.

Expected Outcome

An array of explain objects that have only 1 object.

Failing Test Case / Pull Request

• 🤩 I submitted a PR with a fix and a test case.
• 🧐 I submitted a PR with a failing test case.

Environment

Server

• Parse Server version: 4.5.0 and latest master
• Operating system: docker linux
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): self hosted

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 4.4
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): self hosted

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): Swift SDK
• SDK version: 1.8.3

Logs

[mtrezza]

Thanks for opening this issue. Is this an enhancement or a bug?

[cbaker6]

I would label this as a bug. The reason for this is in JS because the language isn’t strongly typed, but from an implementation standpoint, explain should have always returned an array since every other query returns an array when using find. The bug was discovered because of its use in the Swift SDK which is strongly typed https://community.parseplatform.org/t/explain-result-in-parseswift/1557/2?u=cbaker6

[mtrezza]

Isn't the ObjC SDK also strongly typed? How is that solved there?

[cbaker6]

Isn't the ObjC SDK also strongly typed? How is that solved there?

I can check, but from memory, I don’t think the Objc SDK has the ability to explain

[mtrezza]

I think you are right, I just looked through it.

[mtrezza]

This will break SDKs such as the JS SDK.

Do you mean that a change will inevitable include a breaking change for the JS SDK? Or is there anything broken currently in the JS SDK?

I assume you mean the breaking change of

const query = new Parse.Query('_User');
query.explain();
const result = await query.find();

returning an array instead of an object?

In addition, this brings up another issue in the JS SDK which doesn't support explain for first queries.

Is this related to results not being an array, or a completely separate issue?

[cbaker6]

Do you mean that a change will inevitable include a breaking change for the JS SDK? Or is there anything broken currently in the JS SDK?

Yes, plus, any dev who uses Mongo and explain will have to modify their code no matter if they use JS or or REST directly. I think the Flutter SDK might use explain also in which it will probably break. The only SDK that won’t break is the Swift SDK because I never conformed to the bug

Is this related to results not being an array, or a completely separate issue?

Kind of, first should automatically take the first index of results on the client side. I added a test on this for the server which is why I say the PR in the JS SDK is needed. Using find should return an array.

The easiest way to compare is looking at the implementation in Swift:

First:
https://github.com/parse-community/Parse-Swift/blob/f877b37bf4c4536d41b600791d6cbbae328c7575/Sources/ParseSwift/Types/Query.swift#L1572-L1583

Find:
https://github.com/parse-community/Parse-Swift/blob/f877b37bf4c4536d41b600791d6cbbae328c7575/Sources/ParseSwift/Types/Query.swift#L1564

[mtrezza]

Before we weigh breaking all SDKs vs. making the Swift SDK conform to the "bug" - would it be possible at all to make the Swift SDK conform?

[cbaker6]

Before we weigh breaking all SDKs vs. making the Swift SDK conform to the "bug" - would it be possible at all to make the Swift SDK conform?

Doing this would break the Postgres implementation in which I setup correctly on the server side #6506. In its current form (before this PR), only Mongo or Postgres explain can work on strongly typed SDKs, not both as the Mongo explain was originally coded incorrectly

[cbaker6]

My point is, the Mongo storage adaptor is disobeying the correct sending of results, particularly when using JSON.

when developing the Swift SDK I never conformed to bugs as long as I knew it was a bug originally. Of course, I could have introduced bugs which should be fixed over time.

[mtrezza]

Yes, I understand your point, it makes sense to improve these things. I am trying to understand the implication of such a change. If it breaks only developer code, then this is just another breaking change. But if it breaks SDKs, we may need to prepare these SDKs before making the change in the server. It would be interesting to know which SDKs exactly this would break.

[cbaker6]

Yes, I understand your point, it makes sense to improve these things. I am trying to understand the implication of such a change. If it breaks only developer code, then this is just another breaking change. But if it breaks SDKs, we may need to prepare these SDKs before making the change in the server. It would be interesting to know which SDKs exactly this would break.

From memory, the JS, Swift (non-breaking), and Flutter SDKs have explain. I don’t think the others have it, but I’m on mobile, so we will have to confirm

[cbaker6]

For the JS SDK, this will not break the SDK itself, but breaks it's testcases. In addition, if devs were using find explain they will need to fix their code to use an array instead of an object.

It seems the only SDKs that have the ability to explain are the JS and Swift SDKs. I’ve checked the obj-c, android, flutter, php, and dotNet and can’t find explain in any of them.

[mtrezza]

Then I think we can go ahead with a PR and highlight the breaking change.

[mtrezza]

I classified this as a bug with severity S4 (small / trivial):

• It does not impact any existing functionality, but can be considered a "conceptual flaw" with the effect that it does not allow to easily implement explain in SDKs with strongly typed languages.
• The workaround is to use the REST API and manually parse the response.

[cbaker6]

Another workaround is to write an afterFind trigger in Cloud Code to take the object explain returns and place it in array. This should allow explain to work work with strongly typed SDKs like the Swift SDK.