Call to undefined method Illuminate\Database\Eloquent\Relations\BelongsTo::save()

I can't store the foreign key to my database. This is my reviewController and my foreign key is province_id and category_id. I don't know why I can use save().

public function store(Request $request)
{
    $review = Review::create($request->all());
    if($request->hasFile('cover_image')){
        $cover_filename = $request->file('cover_image')->getClientOriginalName();
        $cover_name = date('Ymd-His-').$cover_filename;
        $public_path = 'img/reviews/';
        $destination = base_path() . "/public/" . $public_path;
        $request->file('cover_image')->move($destination, $cover_name);
        $review->cover_image = $public_path . $cover_name;
    }
    $review->user_id = Auth::user()->id;
    $review->save();
    $provincesId = $request->input('province_list');
    $categoriesId = $request->input('category_list');
    session()->flash('flash_message',  'Create Review Complete.'); 
    $review->province_id=$provincesId;
    $review->category_id=$categoriesId;
    return redirect('/');
}

via Chebli Mohamed