文中的 store 方法里面的 save 方法改成 create 方法为什么会报错呢

public function store(TopicRequest $request, Topic $topic)
{
   //这种写法会报user_id没有默认值的错
    $data = $request->all();
    $data['user_id'] = Auth::id();
    $topic = Topic::create($data);

    //这种写法就可以
    $topic->fill($request->all());
    $topic->user_id = Auth::id();
    $topic->save();
    return redirect()->route('topics.show', $topic->id)->with('message', 'Created successfully.');
}