Make empty changeset updates using .optional() , either ignore or bypass errors QueryBuilderError s.

[dessalines]

Setup

Versions

• Rust: - rustc 1.80.0-nightly (9c9b56879 2024-05-05)
• Diesel: - 2.1.6
• Database: - Postgres 16
• Operating System - Arch

Feature Flags

• diesel: NA

Problem Description

Oftentimes, when doing DB updates, we need to build changeset objects, where we don't know beforehand if its an empty changeset, IE if all the updated fields are None.

When that's the case, diesel throws a runtime QueryBuilderError, which needs to be explicitly caught.

We'd like to either ignore, or bypass that error completely, rather than having to catch runtime QueryBuilderError s for every single update in our application.

The best solution IMO, would be to add this functionality to the existing .optional() , which already does a similar function: turning empty get_result from an error to an option.

Adding this would also turn an empty update / changeset into an Option. IE, if .optional() is done, convert a QueryBuilderError into None, rather than throwing a runtime error.

Context: LemmyNet/lemmy#4705

Also #598 , #1996

Checklist

• I have already looked over the issue tracker and the discussion forum for similar possible closed issues.

• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)
• This issue can be reproduced without requiring a third party crate

[weiznich]

@dessalines I've converted this to a discussion as this is strictly speaking a new feature.
That written: I'm open to accept such an extension as long as it is reasonable named, documented, tested and only matches that single error case (and nothing else).

The .optional() method is just provided by an extension trait implemented on Result that internally matches the error and converts the result. That's something that could also be done in any other crate, so that's not even blocked having it in diesel itself.

[dessalines]

Ignore my last comment, I figured out how to do this with the existing optional.

[dessalines]

k, PR here.