Laravel 5.6不在`created`模型事件中保存字段

Question

I am trying to save a field from the created model event but for some reason the stripe_coupon_id column is never being saved. The created event does run as I've tested by trying a dd inside it and it does fire the event but just does not save that column.

class DiscountRate extends Model
{
    public $table = "discount_rates";

    public $primaryKey = "id";

    public $timestamps = true;

    public $fillable = [
        'id',
        'name',
        'rate',
        'active',
        'stripe_coupon_id'
    ];



    public static function boot()
        {
            parent::boot();
            self::created(function ($discountRate) {
                $coupon_id = str_slug($discountRate->name);
                $discountRate->stripe_coupon_id = $coupon_id;
            });

        }
}

In my controller I simply call a service function which calls the default Laravel model create function:

public function store(DiscountRateCreateRequest $request)
    {
        $result = $this->service->create($request->except('_token'));

        if ($result) {
            return redirect(route('discount_rates.edit', ['id' => $result->id]))->with('message', 'Successfully created');
        }

    }

discount_rates table: