你好，我是新来拉威尔，我想种子表与多种关系这样

for ($i=0; $i < 30; $i++) {
        Product::create([
            'title' => $faker->sentence(1),
            'slug' => $faker->slug,
            'subtitle' => $faker->sentence(3),
            'categorie' => $faker->sentence(1),
            'ville' => $faker->sentence(1),
            'description' => $faker->text,
            'price' => $faker->numberBetween(15, 300),
            'duration' => $faker->numberBetween(15, 300),
            'image' => '//imgur.com/a/WhaAC9O'
        ])->categories()->attach([
            rand(1, 4),
            rand(1, 4)
        ])

但是当我添加这个部分时，它在播种时抛出了一个异常

->villes()->attach([
            rand(1, 1),
            rand(1, 1)
        ])

;
例外情况如下：
对null调用成员函数villes（）
我该怎么办？
这是product.php

class Product extends Model
{

 public function categories()
{
    return $this->belongsToMany('App\Category');
}

public function villes()
{
    return $this->belongsToMany('App\Ville');
}

}