Fix: Incorrectly preloading through association records when middle association has been loaded

[joshuay03]

Fix: #45822

This Pull Request has been created to address #45822.

Update: Looks like this also resolves #45542.

Detail

This Pull Request fixes ActiveRecord::Associations::Preloader::ThroughAssociation#records_by_owner to:

1. successfully fetch the loaded source records for an already loaded middle record
2. do so without performing unnecessary queries

RE: 1
Using a failing test and some logging, I found that the memoized keys in source_records_by_owner had different object_ids to the values in through_records_by_owner at the point at which the value was fetched using source_records_by_owner[record] and was therefore returning nil.

This was being caused by def middle_records loading new objects into @preloaded_records which would get passed in as the records for the source preloader, rather than using the already loaded through objects in through_records_by_owner.

This resulted in the source_records_by_owner hash containing keys which were different objects to the values in the through_records_by_owner hash.

RE: 2
This also happens to fix a circular dependency of sorts, since def middle_records would eventually call def source_preloaders while already in a previous call to it originating from def source_records_by_owner, causing the through record/s to reload and the source record/s to load, before finally loading the source records once again. These extra queries are visible in my comment below.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• CI is passing.

[joshuay03]

This fails without the fix above

[joshuay03]

Down from 3 queries to 1 🎉

[zzak]

FWIW, I've confirmed this fixes the issue described in #45542, but I'm not authoritative enough to say whether or not it's a real bug.

[joshuay03]

FWIW, I've confirmed this fixes the issue described in #45542, but I'm not authoritative enough to say whether or not it's a real bug.

@zzak Thanks! I've updated the PR description with this issue too.

[such]

This looks great! Thanks!

[joshuay03]

Cheers! @byroot

[yopzer]

Hi, Thanks for the fix, this issue was blocking application I work on from upgrading to Rails 7.0. If I understand correctly how rails releases work, this fix will be included in Rails 7.1 release as it was merged in rails:main branch and the work on 7.1 is already in progress. Any chances getting this in 7.0?

[byroot]

I just backported it in 7-0-stable as 99ee8bd.

You can point your Gemfile at this branch.

[yopzer]

Thanks a lot!