morphClass property not set on custom MorphPivot classes

[stancl]

Laravel Version

12.10.0+ from quick testing

PHP Version

8.4

Database Driver & Version

irrelevant

Description

As part of the recent changes to relations, it seems there's a regression in handling polymorphic pivots.

We're seeing $this->morphClass being set to null on a class extending MorphPivot (being used for the relation).

Stack trace (maybe not very relevant):

  ...  my code referencing $this->morphClass inside a deleting() listener
  5   vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php:461
  6   vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php:288
  7   vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php:268
  8   vendor/laravel/framework/src/Illuminate/Events/Dispatcher.php:234
  9   vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/HasEvents.php:224
  10  vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1473
  11  vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/Concerns/AsPivot.php:133
  12  vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/MorphPivot.php:59
  13  vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php:476
  14  vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php:433

This seems to be specifically when detaching.

Haven't had a chance to look into this in depth, but it seems to be caused by #55490, reverting InteractsWithPivotTable to the previous commit (99e1ceb) fixes this.

Steps To Reproduce

Don't have a confirmed minimal repro, this is part of more complex logic, however using a custom MorphPivot (using() on a relation), inside that doing:

public static function booted()
{
    static::deleting(function (self $pivot) {
        dd($pivot->morphClass);
    });
}

should produce null on the latest commit but a class string on 99e1ceb.

[stancl]

@amir9480 If you have time, could you test the reproduction I provided and see if you can tell what in the linked PR could've caused this? Thanks!