When concurrent NotifyRecipientsJobs process an Event and a Mention for
the same user+card, Rails' dirty tracking can skip writing source_type
in the UPDATE if it hasn't changed from the stale in-memory value,
leaving source_type and source_id mismatched (e.g. source_type='Event'
with a Mention's source_id, resulting in a nil source).
Force source_type to always be included in the UPDATE via
source_type_will_change! to prevent this.
Here's a sample timeline of this race condition happening in the real
world (with simplified IDs):
Two `NotifyRecipientsJob` involving notification `03fklpu`, same card,
same user, same comment — enqueued within 50ms of each other:
1. Both jobs load notification `03fklpu` — it has source_type='Mention'
(from a previous job)
2. `EventNotifier` writes at 21:22:36.051: ```sql SET
source_type='Event', source_id=<event_id>, unread_count=1 ```
3. `source_type` included because it changed ('Mention' → 'Event')
4. `MentionNotifier` writes at 21:22:36.057 (~6ms later): ```sql SET
source_id=<mention_id>, unread_count=1 ```
5. No `source_type`! It was `'Mention'` when loaded and `'Mention'` is
what it's setting → not dirty → skipped
6. Final DB state: `source_type='Event'` (from step 2, untouched),
`source_id=<mention_id>` (from step 3)
`Notification.source` now does `Event.find(<mention_id>) → nil.`
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Rosa Gutierrez
95114b1d67