ActiveRecord::InvalidForeignKey: Mysql2::Error: Cannot add or update a child row: a foreign key constraint fails (`exercism`.`user_notifications`, CONSTRAINT `fk_rails_cdbff2ee9e` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT) (ActiveRecord::In...

## Error

```
Cannot add or update a child row: a foreign key constraint fails (`exercism`.`user_notifications`, CONSTRAINT `fk_rails_cdbff2ee9e` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT) (Mysql2::Error)
```

## Stack trace

```
mysql2/client.rb:151 in _query
mysql2/client.rb:151 in block in Mysql2::Client#query
mysql2/client.rb:150 in Thread.handle_interrupt
mysql2/client.rb:150 in query
active_record/connection_adapters/mysql2/database_statements.rb:100 in block (2 levels) in ActiveRecord::ConnectionAdapters::Mysql2::DatabaseStatements#raw_execute
active_record/connection_adapters/abstract_adapter.rb:1027 in block in ActiveRecord::ConnectionAdapters::AbstractAdapter#with_raw_connection
active_support/concurrency/null_lock.rb:9 in synchronize
active_record/connection_adapters/abstract_adapter.rb:999 in with_raw_connection
active_record/connection_adapters/mysql2/database_statements.rb:98 in block in ActiveRecord::ConnectionAdapters::Mysql2::DatabaseStatements#raw_execute
active_support/notifications/instrumenter.rb:58 in instrument
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [ActiveRecord::InvalidForeignKey: Mysql2::Error: Cannot add or update a child row: a foreign key constraint fails (`exercism`.`user_notifications`, CONSTRAINT `fk_rails_cdbff2ee9e` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT) (ActiveRecord::In...](https://thalamus-ai.sentry.io/issues/115037622/)
- **Culprit**: CreateOnboardingNotificationsJob
- **First seen:** Apr 25, 2026