send multiple data and show them in resource api laravel

i have 2 variables that i want to send them to resource and show them in api like below :

        return new RoomDetailResource($data_array,$sum);

and the resource i have written is like below :

class RoomDetailResource extends JsonResource
{
    /**
     * @var
     */
    public $sum;

    /**
     * Create a new resource instance.
     *
     * @param  mixed  $resource
     * @return void
     */
    public function __construct($resource, $sum)
    {
        // Ensure you call the parent constructor
        parent::__construct($resource);
        $this->resource = $resource;

        $this->sum = $sum;
    }

    /**
     * Transform the resource into an array.
     *
     * @param  \Illuminate\Http\Request
     * @return array
     */
    public function toArray($request)
    {
        return [
            'id' => $this->id,
            'sum' => $this->sum
        ];
    }
}

now what i get is the error :

   "message": "Trying to get property 'id' of non-object",
    "status_code": 500,

but if i want to show sum its shows without any problem now here is how i want my api :

{
id:1,
sum:200
},
{
id:2,
sum:200
}

note that sum is the same for all and i just want to repeat it in objects or show them in the end of api as a property . thanks