Cannot update data collection casts that are null by default.

[oddvalue]

✏️ Describe the bug
Cannot update data collections that are null by default.

↪️ To Reproduce

it('can update a model where the cast is initially null', function () {
    $model = new DummyModelWithCasts();
    $model->setRawAttributes(['data_collection' => null]);
    $model->save();
    assertDatabaseHas(DummyModelWithCasts::class, [
        'data_collection' => null,
    ]);

    $model->update([
        'data_collection' => \Illuminate\Support\Collection::make([new SimpleData('Test')]),
    ]);

    assertDatabaseHas(DummyModelWithCasts::class, [
        'data_collection' => json_encode([['string' => 'Test']]),
    ]);
});

✅ Expected behavior
Should be able to update from null

🖥️ Versions

Laravel: 12.x
Laravel Data: 1.17.0
PHP: 8.3

[rseyferth]

I had the same problem in production after an update to 4.17.

It was fixed by modifying the compare method in DataCollectionEloquentCast as follows (quick hacky code):

  public function compare($model, string $key, $firstValue, $secondValue): bool
    {
        $v1 = $this->get($model, $key, $firstValue, []);
        $v2 = $this->get($model, $key, $secondValue, []);
        if (is_null($v1) && !is_null($v2)) return false;
        if (is_null($v2) && !is_null($v1)) return false;
        if (is_null($v1) && is_null($v1)) return true;
        return $v1->toArray() === $v2->toArray();
    }

everything works again after rollback to laravel-data v4.15