//获取已发布的100条文章
$posts = Post::limit(100)->get(); //一次查询

$authors = array_map(function($post) {
 // 对作者模型生成查询
 return $post->author->name;
}, $posts);

//获取已发布的100条文章 - 并预加载文章对应作者
$posts = Post::with('author')->limit(100)->get();//2次查询

$authors = array_map(function($post) {
 // 对作者模型生成查询
 return $post->author->name;//这里讲不在产生查询
}, $posts);

select * from `posts`
select * from `authors` where `authors`.`id` in (?, ?, ?, ?, ?) [1,2,3,4,5]

$posts = App\Post::with(['author', 'comments'])->get();

Post -> belongsTo -> Author //每个文章只属于一个用户
Author -> hasMany -> Post //每个用户拥有多个文章
Author -> hasOne -> Profile //每个用户只有一个简介

//获取所有文章 - 并预加载文章对应作者
$posts = App\Post::with('author')->get();//两次查询

//根据每个 `作者` 获取其简介
$posts->map(function ($post) {
 //虽然我们直接通过$author = $post->author不会产生查询，
 //但当调用$author->profile时，每次都会产生一个新查询
 return $post->author->profile;
});

//获取所有文章 - 并预加载文章对应作者及每个作者对应de profile
$posts = App\Post::with('author.profile')->get();//三次查询

$posts->map(function ($post) {
 //不在产生新查询
 return $post->author->profile;
});

select * from `posts` 
select * from `authors` where `authors`.`id` in (?, ?, ?, ?, ?) [.....] 
select * from `profiles` where `profiles`.`author_id` in (?, ?, ?, ?, ?) [.....] 

$posts = App\Post::all();//一次查询

$posts->load('author.profile');//两次查询
$posts->map(function ($post) {
 //不在产生新查询
 return $post->author->profile;
});