A has_many :through writer on a persisted owner writes its join rows at once

[alextakitani]

Probed on main (26365fa), Linux x86-64, CRuby 4.0.5.

The synthesized has_many :through collection writer only stages the collection and marks it stale; _sync_<name> writes the join rows from after_save. Rails writes them immediately when the owner is already persisted, and defers only for a new record. So assigning after the save is lost:

class Article < ApplicationRecord
  has_many :labelings, dependent: :destroy
  has_many :labels, through: :labelings
end

article = Article.create!(title: "x", body: "Long enough body")
article.labels = [a, b]
Labeling.count

	Rails	main
Labeling.count after assigning on a saved record	2	0
reading article.labels back	[a, b]	[a, b] (from the in-memory cache)

The read-back hides it: the reader returns the staged cache, so the record looks tagged and nothing reports the missing rows. Found transpiling a time tracker whose controller saves the entry, then assigns its tags; the JSON response listed the tags and the taggings table stayed empty.

Fix: the writer calls self._sync_<name> when self.persisted? (the same self.persisted? spelling markers.rs uses, since it is synthesized per model on every target). A new record keeps the existing deferral to after_save.

Test: tests/through_writer_persisted_owner.rs::a_through_collection_writer_on_a_persisted_owner_writes_at_once covers both halves: persisted owner writes at once, a replace leaves one row, and a new owner writes nothing until save!. It fails on main with persisted owner: 0 join rows, want 2.

Ran: emit_and_run (108), lowered_ruby_emit (108), real_blog (6), model_lowerer (27), polymorphic_associations (6), assoc_relation_seed (7), spinel_blog_library (4).

Summary by CodeRabbit

• Bug Fixes
  • Assigning a collection through a join relationship now synchronizes join rows immediately for saved records. New records continue to synchronize after saving.
  • Added regression coverage for both saved and new records.

Update: the regression test now lives in its own file, tests/through_writer_persisted_owner.rs (same emit_and_run harness), so it no longer conflicts with other changes appending to tests/emit_and_run.rs. Rebased onto current main.

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• tests/through_writer_persisted_owner.rs

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📝 Walkthrough

Walkthrough

The through collection writer now synchronizes join rows immediately when the owner is persisted. For new owners, synchronization remains deferred until save. A regression test covers both paths and replacement of assigned labels.

Changes

Through collection writer

Layer / File(s)	Summary
Persisted-owner synchronization src/lower/model_to_library/associations.rs, tests/through_writer_persisted_owner.rs	The writer calls _sync_<name> after staging the collection when the owner is persisted. New owners continue to defer synchronization until save. The regression test checks immediate join-row creation and replacement for persisted owners, and deferred creation for new owners.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: 🟡 Moderate · up to 8099d

Assigning an invalid replacement collection can silently remove previously saved associations. Resolve this failure path before merging unless that data-loss behavior is explicitly accepted.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 8099d

Immediate writes fix the reported loss of associations after saving. However, replacement can delete existing associations before a new join row fails validation, without reporting the failure or scheduling recovery. Operations remain scoped to the owner; no authorization bypass has been established.

Retained concerns

• Medium · reliability · inferred: Persisted-owner assignment now reaches a replacement sequence that clears stale state and destroys existing join rows before saving replacements. A rejected replacement save can leave missing or partial associations while the loaded cache still represents the requested collection. The failure is not propagated, and later owner saves do not automatically retry. This replacement weakness existed at save time before the PR; the PR expands its exposure to assignment-time mutations.

Security review details

Security Blast Radius

• inferred — The established propagation scope is generated writers for resolved through associations. A replacement directly selects join rows for one owner ID. Broader effects depend on application declarations and destroy callbacks; the supplied evidence does not establish tenant-wide exposure or permission-bearing associations.

Trust Boundaries and Controls

• observed — The persistence-state guard decides when writes occur; it is not an authorization check. Owner identity comes from the receiver, and replacement target identities come from supplied objects. The inspected entrypoint supplies saved fixture objects and establishes no attacker-controlled request path.

Resilience and Maintainability Implications

• inferred — An explicit caller transaction can roll back an exception, but does not by itself contain a silently rejected join save: the runtime commits normal returns, and synchronization ignores the rejection. Database rollback also does not establish restoration of the already-cleared in-memory stale state.

Hardening Proposals

• proposed — Make replacement an atomic operation with explicit save-failure propagation. Clear stale state only after successful completion, and restore or invalidate the association cache on failure so database and in-memory recovery remain aligned.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the main change: a has_many :through writer immediately writes join rows when its owner is persisted.

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Actionable comments posted: 2

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/lower/model_to_library/associations.rs:
- Line 1398: Update the generated _sync_<name> method in the association
generation flow to diff current and requested targets, deleting join rows only
for removed targets and preserving rows for targets that remain assigned. Avoid
destroying and recreating retained rows so their IDs, timestamps, and
join-specific attributes remain unchanged.
- Line 1398: Update the generated _sync_<name> method to replace join rows
inside a transaction, check every join-row save and propagate failure so the
replacement rolls back, and clear the stale flag only after all rows save
successfully.

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• src/lower/model_to_library/associations.rs
• tests/emit_and_run.rs

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