Laravel 5: Validate if field value exists if data source is not MYSQL but Repository

I am working on REST API with add/update/delete methods.

In Controller, In Update action i have to check if id passed exists, I know i can use Laravel validator exists:table,column , but i am using Repository inside controller

public function __construct(BookRepositoryInterface $bookRepository, Validator $validator)
{
    $this->bookRepository = $bookRepository;
    $this->validator = $validator;
}

Repository source can be different than mysql database. Laravel validator 'exists' will not work in every case.

This is the code for updating book by an id.

public function update($id)
{
    $params = Input::only('title', 'author');

    $params['id'] = $id;

    $rules = array(
        'id' => 'required',
        'author' => 'required',
        'title' => 'required'

    );

    $validator = $this->validator->make($params, $rules);

     ...........
}

What is best way to check if id exists in book repository during update?

via Chebli Mohamed