验证失败后突出显示多个选择选项-Laravel 5.2

我是第一次学习Laravel 5.2，当验证失败时，我需要重新填充表单。

我可以用"name"或"description"这样的单一输入来完成，但我找不到如何用多选的"category"来完成。

视图：

<form action="{{ url('addproduct') }}" method="POST">
                {!! csrf_field() !!}
                <div class="form-group">
                    <label for="#productName">Name:</label>
                    <input id="productName" class="form-control" name="name" type="text" placeholder="Product name" value="{{ old('name') }}">
                </div>
                <div class="form-group">
                    <label for="#productDescription">Description:</label>
                    <textarea id="productDescription" class="form-control" name="description" rows="3" placeholder="Product description" value="{{ old('description') }}"></textarea>
                </div>
                @if ($categories)
                <div class="form-group">
                    <label for="#productCategory">Category:</label>
                    <select id="productCategory" class="form-control" name="category[]" multiple>
                      @foreach ($categories as $category)
                      <option value="{{ $category->id }}">{{ $category->name }}</option>
                      @endforeach
                    </select>
                </div>
                @endif
                <div class="form-group">
                    <label for="#productQuantity">Quantity:</label>
                    <input id="productQuantity" class="form-control" name="quantity" type="number" min="0" value="0">
                </div>
                <button class="form-control btn btn-success" type="submit">Add Product</button>
            </form>

控制器：

    public function add(Request $request) {
    $data = array();
    $data['msgAdded'] = false;
    $data['categories'] = Category::select('name', 'id')->get();
    if ($request->isMethod('post')) {
        $this->validate($request, [
            'name' => 'required|max:50',
            'description' => 'required|max:200',
            'category' => 'required',
            'quantity' => 'required|min:0',
        ]);
        $product = new Product;
        $product->name = $request->name;
        $product->description = $request->description;
        $product->quantity = $request->quantity;
        $product->save();
        foreach ($request->category as $cat) {
            $category = Category::find($cat);
            $category->products()->attach($product->id);
        }
        $data['msgAdded'] = true;
    }
    $data['view'] = 'addproduct';
    return view('products/add', $data);
}

我想知道它是否可以用刀片实现，或者实现这一点的最佳方式。

谢谢！