MONGOID-5881 Persist empty array set via raw attribute write in before_save

[jamis]

Description

Fixes MONGOID-5881: setting an embeds_many association to an empty array via a raw attribute write (e.g. self[:labels] ||= []) in a before_save callback was not being persisted to the database.

Root cause

The regression was introduced by the MONGOID-4397 fix (commit e08fff0e5, Mongoid 8.0+). That commit added update_attributes_hash to EmbedsMany::Proxy, which deletes the association key from the parent's attributes hash whenever the proxy target is empty:

def update_attributes_hash
  if _target.empty?
    _base.attributes.delete(_association.store_as)  # ← the problem
  else
    _base.attributes.merge!(...)
  end
end

On insert, as_attributes iterates over all embedded relations and calls send(:labels) to lazily initialize each proxy. When the proxy initializes with an empty target (from the [] the callback just wrote), update_attributes_hash deletes the key. add_attributes_for_relation then sees neither the key nor a non-blank relation, returns early, and the insert document is assembled without labels: [].

On update, the bug does not trigger because generate_atomic_updates (which reads changes) runs before _descendants.each (which lazily initializes the proxy), so the dirty state is captured while the key is still present.

Fix

In update_attributes_hash, only delete the key when it is absent or nil. If the key is already [], a caller explicitly set it to an empty array and that intent must be preserved:

_base.attributes.delete(stored) unless _base.attributes[stored] == []

Tests

Two new examples under when trying to persist the empty list → when set via raw attribute write in a before_save callback:

• on a new record — the previously failing INSERT case
• on an existing record — the UPDATE case with a freshly-loaded document (proxy uninitialized at save time)

The existing does not persist the empty list example (covering the never-set case) continues to pass, confirming the fix does not cause spurious empty arrays to appear in documents that never had the association set.

[Copilot]

Pull request overview

Fixes a regression where an embeds_many set to an empty array via a raw attribute write in a before_save callback could be dropped during inserts, causing [] not to be persisted.

Changes:

• Adjust EmbedsMany::Proxy#update_attributes_hash to preserve an explicitly-set empty array in the parent’s attributes.
• Add regression specs covering raw attribute writes in before_save for both inserts and updates.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/mongoid/association/embedded/embeds_many/proxy.rb	Updates proxy attribute-hash synchronization to avoid deleting explicitly-set empty arrays.
spec/mongoid/association/embedded/embeds_many/proxy_spec.rb	Adds regression coverage for persisting [] when set via raw attribute write in a before_save callback.

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