Laravel：`loadMissing` 函数的目的是什么

很好的问题；有些细微的差异并不是通过阅读文档就能立即反映出来的.

You are comparing "Lazy Eager Loading" using loadMissing() to "Lazy Loading" using magic properties on the model.

The only difference, as the name suggests, is that:

• "延迟加载"只在关系使用时发生.
• "Eager lazy loading" can happen before the usage.

所以，实际上，没有区别，除非您想在使用关系之前显式地加载它.

还值得注意的是，load和loadMissing方法都为您提供了定制关系加载逻辑的机会，方法是传递闭包，而在使用magic属性时，闭包不是选项.

$book->loadMissing(['author' => function (Builder $query) {
    $query->where('approved', true);
}]);

Which translates to "Load missing approved author if not already loaded" which is not achievable using $book->author unless you define an approvedAuthor relation on the model (which is a better practice, though).

直接回答你的问题；是的，如果你删除了:

$book->loadMissing('author'); 

in that particular example as it's being used right after the loading. However, there might be few use cases where one wants to load the relation before its being used.

因此，要概述关系加载方法是如何工作的:

急装

通过使用with()，您可以在查询父模型时"立即加载"关系:

$book = Book::with('author')->find($id);

Lazy eager loading

为了紧急加载关系after，父模型已经被检索:

$book->load('author');

它还可以用于仅急于加载丢失的文件的方式:

$book->loadMissing('author');

与load()方法相反，loadMissing()方法过滤给定的关系并懒惰地"Eager "地加载它们only if not already loaded.

通过接受闭包，这两种方法都支持自定义关系加载逻辑.

懒惰加载

通过使用magic properties实现的延迟加载是为了方便开发人员.它在使用关系时加载关系，因此您不需要事先加载它.

@rzb在his answer篇文章中也提到了一个非常好的观点.看一看