Laravel预加载

当通过属性访问 Eloquent 关联时，该关联数据会被「延迟加载」。意味着该关联数据只有在你使用属性访问它时才会被加载。
例如

$books = App\Book::all();

foreach ($books as $book) {
    echo $book->author->name;
}

这样将执行N+1步查询：
1次：查询出所有书N本，
N次：每本书查询一次作者

预加载的话

$books = App\Book::with('author')->get();

foreach ($books as $book) {
    echo $book->author->name;
}

只执行两部

select * from books

select * from authors where id in (1, 2, 3, 4, 5, ...)

预加载的方式

加载多个关联
  $books = App\Book::with('author', 'publisher')->get();

加载嵌套关联
    $books = App\Book::with('author.contacts')->get();

筛选加载结果
      $users = App\User::with(['posts' => function ($query) {
           $query->where('title', 'like', '%first%');
           }])->get();

延迟预加载
    $books = App\Book::all();

     if ($someCondition) {
          $books->load(['author' => function ($query) {
             $query->orderBy('published_date', 'asc');
      }]);
}