whenLoaded() 是 Laravel 中用于條件性處理已加載關(guān)系的方法。它會(huì)在指定關(guān)系已被加載時(shí)執(zhí)行回調(diào)函數(shù)，避免未加載時(shí)觸發(fā) N 1 查詢或錯(cuò)誤。例如在 API 資源中僅當(dāng)關(guān)系存在時(shí)才添加字段、在不強(qiáng)制加載的前提下處理關(guān)聯(lián)數(shù)據(jù)、結(jié)合 optional() 安全訪問(wèn)屬性。使用時(shí)需確保關(guān)系名準(zhǔn)確且已通過(guò) with() 或動(dòng)態(tài)訪問(wèn)預(yù)先加載，閉包接收關(guān)系結(jié)果（模型或集合），適用于優(yōu)化性能和數(shù)據(jù)轉(zhuǎn)換。

In Laravel, when you're working with Eloquent relationships and need to conditionally load or add a relationship to a model based on whether it was loaded from the database, whenLoaded() is a useful method. It helps avoid unnecessary data fetching and keeps your code clean by checking if a relationship has already been loaded.

Here’s how and why you’d use it.

What does whenLoaded() actually do?

The whenLoaded() method checks if a specific relationship was loaded on a model. If it was, it lets you run a callback that modifies or augments that relationship.

For example:

$user->whenLoaded('profile', function ($profile) {
    // This only runs if the profile relationship was loaded
    return $profile->is_complete ? 'Complete' : 'Incomplete';
});

This avoids trying to access $user->profile when it might not be available (like in cases where it wasn’t eager-loaded), which could lead to N 1 query issues or errors.

When should you use whenLoaded()?

You’ll typically reach for whenLoaded() in scenarios like:

• Transforming API responses — only include extra fields if related models are present.
• Avoiding N 1 queries — only process relationships that were already eager-loaded.
• Conditional logic in resource collections or transformers — adjust output based on what data is available.

Let’s say you have a user resource that sometimes includes their posts. You want to count comments only if posts are loaded:

$user->whenLoaded('posts', function ($posts) {
    return $posts->flatMap->comments->count();
});

That way, you’re not forcing a load of posts just to count comments.

How to use whenLoaded() in real code

Here’s a more practical example inside a Laravel resource collection:

public function toArray($request)
{
    return [
        'id' => $this->id,
        'name' => $this->name,
        'email' => $this->email,
        'latest_post_title' => $this->whenLoaded('posts', function ($posts) {
            return optional($posts->first())->title;
        }),
    ];
}

A few notes here:

• We’re using optional() to safely access the first post's title without throwing an error if there are no posts.
• The latest_post_title key will only appear if the posts relationship was loaded.

If you don’t use whenLoaded(), you might end up with either missing data or unexpected queries being triggered behind the scenes.

A few gotchas and tips

• Make sure the relationship name passed to whenLoaded() matches exactly — it's case-sensitive and must match the method name on the model.
• whenLoaded() doesn't load the relationship for you — it only acts if it was already loaded via with() or dynamically accessed earlier.
• It works great in combination with Laravel's conditional attributes (when() and mergeWhen()).

Some common mistakes:

• Trying to use whenLoaded() expecting it to lazy-load the relation — it won’t.
• Forgetting that the closure receives the relation result directly (i.e., a Collection or Model, depending on the type of relationship).

So if you're working with a hasOne or belongsTo, you'll get a single model or null. With hasMany, you'll get a collection.

Using whenLoaded() properly can make your code cleaner and more efficient, especially when building APIs or complex data transformations. It's one of those small tools in Laravel that feels simple but makes a big difference in avoiding performance pitfalls.

基本上就這些。

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