I'm new to Laravel so sorry for my newbie question:

I have in input field, it must be valid only if it's zero or if it's value exists in a "products" db table.
I'm using Form Request Validation with Laravel 5.2.

This is my code:

```
class CreateRequest extends Request
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
      return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
          'product_id'    =>  'sometimes|exists:products,id'
        ];
    }
}

```

This works for the database validation, but how can I make it valid if it's zero?

Thanks for your help!