Laravel雄辩地获得相关表的最新行 [英] Laravel eloquent get the latest row of related table

问题描述

我有两个表，书和章。一本书有很多章。

I have two tables, books, and chapters. One book has many chapters.

书模型：

public function chapters() {
   return $this->hasMany(Chapter::class);
}

章节模型：

public function book() {
   return $this->belongsTo(Book::class);
}

我想使用像这样的单个查询来获取带有自己最新章节的图书清单：

I want to get book list with their own latest chapter using single query like this:

$books = Book::with(['authors', 'categories', 'chapters' => function($q) {
   $q->orderBy('updated_at', 'desc')->first();
}]->get();

但是它不起作用。各章返回一个空数组。如果我删除了 first（）子查询，它可以正常工作。

But it doesn't work. Chapters return an empty array. If I remove first() in the subquery, it works properly.

有什么方法可以只执行一个查询吗？我不想让所有相关的章节都保留一章，或者使用多个我感觉更好的唯一方法是使用 join ，对吗？

Are there any way to do this with just one query. I don't want to get all related chapters then keep one, or using multiple queries. The only way I feel better is using join, is it right?

任何帮助将不胜感激。

推荐答案

由于关系是单独的查询，因此无法为急切加载的关系添加限制，如果这样做，则会限制整个关系查询，它不会成为每个相关对象的限制。

Because relationships are separate queries, you can't add a limit to an eager loaded relationship. If you do, this limits the entire relationship query, it does not act as a limit per related object.

幸运的是，您的要求可以简单地通过附加关系来实现：

Luckily, your requirement can be implemented simply with an additional relationship:

public function latestChapter() {
    return $this->hasOne(Chapter::class)->latest();
}

现在，而不是急于加载整个章关系，您可以只希望加载新的 latestChapter 关系。

Now, instead of eager loading the entire chapters relationship, you can just eager load your new latestChapter relationship.

$books = Book::with(['authors', 'categories', 'latestChapter'])->get();

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