Laravel Backpack添加/更新数据透视表值

public function roles()
{
    return $this->belongsToMany(Role::class);
}

php artisan make:migration create_role_user_table --create=role_user

public function up()
{
    Schema::create('role_user', function (Blueprint $table) {
        $table->unsignedBigInteger('role_id');
        $table->unsignedBigInteger('user_id');
        // 其他附加字段
        $table->timestamps();

        $table->foreign('role_id')->references('id')->on('roles')->onDelete('cascade');
        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
    });
}

use Illuminate\Database\Seeder;
use App\User;
use App\Role;

class RoleUserTableSeeder extends Seeder
{
    public function run()
    {
        $user = User::find(1);
        $role = Role::find(1);

        $user->roles()->attach($role);
    }
}

php artisan db:seed --class=RoleUserTableSeeder