我读过https://laravel.com/docs/5.7/eloquent-relationships#eager-loading的文件，但仍有困难。

为什么此函数的ther分支不返回带有附加的\App\Models\Client对象？

/**
 * 
 * @param \App\Models\Contact $contact
 * @param string $msg
 * @return \App\Models\Customer
 */
public function createCustomerIfNecessary($contact, &$msg) {
    if ($contact->customer) {
        return $contact->customer;
    } else {
        $customer = new \App\Models\Customer();
        $customer->setUuid();
        $customer->contact_id = $contact->id;
        $customer->save();
        $customer->loadMissing('contact');
        $msg .= ' Created new customer with ID ' . $customer->id . '.';
        return $customer;
    }
}

这是它的测试，目前失败（ErrorException:试图在$resultCustomer上获取非对象的属性“id”）：

public function testCreateCustomerIfNecessary() {
    $service = new \App\Services\OauthService();
    $msg = '';
    $contact = new \App\Models\Contact();
    $contactId = 654;
    $contact->id = $contactId;
    $resultCustomer = $service->createCustomerIfNecessary($contact, $msg);
    $this->assertEquals($contactId, $resultCustomer->contact->id);
}

另外，这种关系在我的代码的其他部分也起作用。

联系人模型具有：

public function customer() {
    return $this->hasOne('App\Models\Customer');
}

客户模型具有：

public function contact() {
    return $this->belongsTo('App\Models\Contact');
}

“customers”表有一个“contact_id”外键。