unique exception

/** ROUTE **//
Route::put('post/{post:slug}', 'PostController@update')->name('post.update');

/** CONTROLLER **//
public function update(validateFormPost $request, Post $post)
    {

        $post->judul = $request->judul;
        $post->slug = $request->slug;
        $post->isiBerita = $request->isiBerita;
        $post->categoryId = $request->category;

        $post->update();
        $alertClass = 'success';
        $message = 'Data Post Berhasil dirubah';

        return redirect('post')->with(['message' => $message, 'alertClass'=>$alertClass]);
    }

/** VALIDATION RULES *//

return [
            'slug' => 'required|alpha_dash|unique:posts,slug,'.$this->id,

        ];