MONGOID-5972 Avoid merging conflicting touch paths into embedded inserts

[jamis]

MONGOID-5972: Inserting an embeds_many child while another child of the same array has a pending touch update raises Mongo::Error::OperationFailure [40].

Description

Fixes a regression introduced by MONGOID-5867 (Merge touch updates into embedded document insert). insert_as_embedded merged the parent's pending touch updates into the same update_one that performed the embedded insert. For embeds_many, the insert produces $push on the array while a dirty sibling can make the touches include a path inside that array (e.g. items.0.updated_at). MongoDB rejects a single update whose operators target both a path and one of its ancestors or descendants, so the write failed with error 40. Before 9.1.0 the touch was persisted in its own update_one and the two never shared an update document.

The merge is now skipped when any touch path conflicts (equal, ancestor, or descendant) with a path the insert already targets. Such touches are deferred to the after_save callback's separate round-trip, matching pre-9.1 behavior. The single-round-trip optimization in MONGOID-5867 is preserved for the common case.

Verification

• Added a regression context under describe 'touch merged with embedded insert' in spec/mongoid/touchable_spec.rb (the embedded push/create touch-merge coverage). It fails on master with error 40 and passes with the fix.
• Touchable suite: 245 examples, 0 failures.
• Persistable suite: 881 examples, 0 failures.
• Creatable + touchable suites: 315 examples, 0 failures.
• Timestamps, atomic, and embedded-association suites: 1584 examples, 0 failures.

[Copilot]

🟡 Changes recommended

Two moderate review comments remain unresolved in the regression spec.

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Pull request overview

Fixes MongoDB path conflicts when inserting an embeds_many child alongside pending touch updates.

Changes:

• Detects conflicting touch paths before merging updates.
• Defers conflicting touches to the callback round-trip.
• Adds regression coverage for MONGOID-5972.

File summaries

File	Summary	Review comments
spec/mongoid/persistable/creatable_5972_spec.rb	Adds regression coverage for sibling insertion and touch persistence.	Moderate (2 votes): deregister dynamically defined models during teardown. Moderate (1 vote): assert the sibling’s updated_at after reload.
lib/mongoid/persistable/creatable.rb	Adds equal, ancestor, and descendant touch-path conflict detection.	No comments.

Review details

Suppressed comments (1)

spec/mongoid/persistable/creatable_5972_spec.rb:59

• Could this regression test also assert the first item's updated_at after reload? The new branch deliberately skips merging the $set, so the key behavior is that the after-save fallback still persists the touch; the current qty/archived assertions would pass even if that deferred touch were lost.

    it 'persists the updated first item' do
      first = parent.reload.m5972_items.first
      expect(first.qty).to eq(2)
      expect(first.archived).to be(false)
    end

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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