convert model to array and return as json

How do I get a model from the database and then convert it to an Array including extra information using the with statement.

public function edit($id) {
    // convert product to array;
    $product = Product::findOrFail($id)->with('supplier', 'category');

    $data = [
        'suppliers' => Supplier::all()->pluck('company', 'id'),
    ];        

    // cannot merge because $product is object and cannot turn into array
    // the only way I know to convert to array is doing this
    // $product->first()->toArray() but this gets the first item in the database
    $product = array_merge($product, $data);

    return response()->json($product, 200, ['Content-Length' => strlen(json_encode($product))]);
}

via ONYX