Laravel添加车辆API验证报BadMethodCallException错误求助

我正在开发一个汽车经销商的API（小型学校项目），在CarController处理添加车辆的POST请求时遇到数据验证问题。

接收的示例JSON数据：

{
  "model": "Q5",
  "brand": "Audi",
  "year": 2008,
  "price": 44000,
  "color": "blue",
  "traction": "4x4",
  "motor": {
    "type": "gasoline",
    "hp": 256,
    "turbo": false,
    "cylinders": 4,
    "motor_liters": 2.0
  },
  "user_id": 1
}

我编写的验证规则：

$request->validate([
    'model' => 'required|string',
    'brand' => 'required|string',
    'year' => 'required|integer',
    'price' => 'required|numeric',
    'color' => 'required|string',
    'traction' => 'required|string',
    'motor' => [
        'type' => 'required|string',
        'hp' => 'required|integer',
        'turbo' => 'required|boolean',
        'cylinders' => 'required|integer',
        'motor_liters' => 'required|numeric'
    ],
    'user_id' => 'required|integer'
]);

发起请求后，Validator抛出错误：

BadMethodCallException: Method Illuminate\Validation\Validator::validateRequired|string does not exist.

我不确定是拼写错误、格式问题还是嵌套JSON的处理方式有误，我的UserController中的POST请求验证运行正常：

$request->validate([
    'username' => 'required|string|min:5',
    'password' => 'required|string'
]);

问题出在嵌套字段motor的验证规则写法上。Laravel验证嵌套数组时，必须先给motor字段指定基础验证规则（比如required|array），再定义其内部字段的验证规则，否则框架会把内部字段的规则当成motor字段的验证规则，导致调用不存在的验证方法。

以下两种正确写法任选其一：

写法一：使用点语法 #

$request->validate([
    'model' => 'required|string',
    'brand' => 'required|string',
    'year' => 'required|integer',
    'price' => 'required|numeric',
    'color' => 'required|string',
    'traction' => 'required|string',
    // 先验证motor是必填数组
    'motor' => 'required|array',
    // 用点语法定义motor内部字段规则
    'motor.type' => 'required|string',
    'motor.hp' => 'required|integer',
    'motor.turbo' => 'required|boolean',
    'motor.cylinders' => 'required|integer',
    'motor.motor_liters' => 'required|numeric',
    'user_id' => 'required|integer'
]);

写法二：嵌套数组规则 #

$request->validate([
    'model' => 'required|string',
    'brand' => 'required|string',
    'year' => 'required|integer',
    'price' => 'required|numeric',
    'color' => 'required|string',
    'traction' => 'required|string',
    'motor' => [
        'required',
        'array',
        // 嵌套定义内部字段规则
        'type' => 'required|string',
        'hp' => 'required|integer',
        'turbo' => 'required|boolean',
        'cylinders' => 'required|integer',
        'motor_liters' => 'required|numeric'
    ],
    'user_id' => 'required|integer'
]);

内容的提问来源于stack exchange，提问作者Alex Serafini