We have code in a JsonModel like

public function setZipAttribute($value): void
    {
        // Null will fail validation but the intent can be inferred to mean "delete"
        if (null === $value) {
            unset($this->attributes['zip']);
            return;
        }
        $this->attributes['zip'] = $value;
    }

We're using this so that PATCH requests can send {"zip":null} and it's interpreted to mean "remove the ZIP" or "go back to default ZIP behavior for this account"

We could use cast classes for this, but \Illuminate\Database\Eloquent\Concerns\HasAttributes::setAttributeMarkedMutatedAttributeValuemakes it impossible (for everything I've tried) for any return value of a cast to unset an attribute entirely. (This is because Eloquent is by default thinking about structured relational databases, there's no sane reason to remove a column from the UPDATE query, that won't unset it, that will fail to modify it at all, so PHP null and MySQL null are good compliments, in a way that PHP null and Json "attribute is missing" are not.)

I'd like to try a new cast on JsonModels like "nullUnsets" and encapsulate that behavior into JsonModel::castAttribute (which otherwise inherits from \Illuminate\Database\Eloquent\Concerns\HasAttributes::castAttribute)

If there's a downside of this approach, its that Laravel doesn't have a reason to stack casts, so we'd have to fragment this further like stringOrUnset or floatOrUnset if that becomes important later.