Fixed #35442 -- Prevented N+1 queries in RelatedManager with only().

[Samriddha9619]

ticket-35442

Branch description

This PR prevents N+1 queries when iterating over a RelatedManager queryset that uses .only() to exclude the foreign key field.

Previously, ModelIterable would attempt to optimize the relationship population by accessing the foreign key on the child object. If that key was deferred (via .only()), accessing it triggered an immediate SQL query for every row.

The fix modifies ModelIterable to check obj.__dict__ first. If the foreign key attributes are missing (deferred), the optimization is skipped, preserving the lazy loading behavior requested by the user.

Tests:

• Added test_only_related_manager_optimization in defer.tests to verify query count is 1 (previously N+1).
• Updated defer.tests.DeferTests.test_only to assert the correct number of deferred fields (3 instead of 2), reflecting that the foreign key is now correctly deferred.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

[jacobtylerwalls]

Thanks for the PR! Since this uses Simon's patch from the ticket comments, please add him as a co-author in the commit message. You can grep the commit log to see how this is done. Also please use two hyphens (not one) in the commit message & PR title. Cheers!

[Samriddha9619]

@jacobtylerwalls Done

[jacobtylerwalls]

I think you need another newline before the co-author for the UI to reflect it here (as well as to follow the existing format.)

[Samriddha9619]

I think you need another newline before the co-author for the UI to reflect it here (as well as to follow the existing format.)

Fixed. Thanks for pointing that out.

[github-actions]

📊 Coverage Report for Changed Files

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Diff Coverage
Diff: origin/main...HEAD, staged and unstaged changes
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django/db/models/query.py (100%)
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Total:   3 lines
Missing: 0 lines
Coverage: 100%
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