Question

我有2个模型，例如Country和City，它们之间的关系为one-to-many。

当我运行这段代码

$countries = Country::query()->with('cities')->get();

foreach ($countries as $country) {
    $cities = $country->cities;

    foreach ($cities as $city) {
        dump($city->country);
    }
}

，在每个$city->country调用中，我都有一个查询到数据库

select * from `countries` where `countries`.`id` = ? limit 1

如果没有立即加载关系（在我的情况下为cities.country），是否有可能防止在每个$city->county调用中进行数据库查询？

我不需要$country变量。如果在主查询中未加载关系null，则需要获取cities.country。

Answer 1

为什么不做逆运算：

$cities = City::with('country')->get();

// this way you would loop only once:

foreach ($cities as $city) {
    dump($city->country);
}

然后使用您的方法：

foreach ($countries as $country) {
    $cities = $country->cities;

    foreach ($cities as $city) {
        dump($country);
        // here the $country variable 
       // is the same as the $city->country one, so no need to query it again..
    }
}

Answer 2

感谢

https://laracasts.com/discuss/channels/laravel/how-to-check-if-an-relationship-has-been-eager-loaded-or-not

就我而言

//...
if ($city->relationLoaded('country')) {
    dump($city->country);
} else {
    dump(null);
}
//...

如果未加载关系则阻止数据库查询

2 个答案: