数据关联的疑惑

在 tests/Unit/QuestionTest.php 中：

/** @test */
public function a_question_has_many_answers()
{
    $question = factory(Question::class)->create();

    factory(Answer::class)->create(['question_id' => $question->id]);

    $this->assertInstanceOf('Illuminate\Database\Eloquent\Relations\HasMany', $question->answers());
}

而在 database/factories/AnswerFactory.php 中：

$factory->define(Answer::class, function (Faker $faker) {
    return [
        'user_id' => function () {
            return factory(User::class)->create()->id;
        },
        'question_id' => function () {
            return factory(Question::class)->create()->id;
        },
        'content' => $faker->text
    ];
});

我的疑问是在 tests/Unit/QuestionTest.php 中的：

$question = factory(Question::class)->create();

和 database/factories/AnswerFactory.php 的

'question_id' => function () {
    return factory(Question::class)->create()->id;
},

是怎么对应上的，工厂类里是重新创建一个实例的吧，那感觉单元测试类里面就没必要传 ['question_id' => $question->id] 了。