如何在Laravel的额外字段中使用伪造的值作为pivot数据库表的种子？

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

Schema::create('role_user', function (Blueprint $table) {
    $table->unsignedBigInteger('role_id');
    $table->unsignedBigInteger('user_id');
    $table->string('extra_field')->nullable();
    $table->timestamps();

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

$user->roles()->attach($role, ['extra_field' => factory(App\ExtraModel::class)->create()->id]);