控制器内部雄辩的模型“ with”调用

Question

Let's say I have a Post model that hasMany Comment model. So basically to get the comments I would do $post->comments .

On some websites, I have seen people do this in the controller:

$post = App\Post::with('comments')->findOrFail($id);

return view('someview', compact('post'));

and then inside the view:

@foreach($post->comments as $comment) ...

To my understanding, $post->comments would always have the comments attached and there's no need to call with('comments') . Is this not correct?

If so, then what is the difference between the above and the below:

Controller

$post = App\Post::findOrFail($id);

return view('someview', compact('post'));

View

@foreach($post->comments as $comment) ....

Answer 1

It's called Eager Loading: http://laravel.com/docs/5.1/eloquent-relationships#eager-loading

Eager loading will have no affect on your example. Here is a better use case:

$posts = App\Post::orderBy('id')->take(10)->get();

foreach ($posts as $post)
{
    echo $post->comments;
}

Eloquent will create 11 queries to the database which is not good. 1 query to get the posts, and 10 queries to get the comments for each post.

When you eager load your relation(s), Eloquent will only make one other query.

$posts = App\Post::with('comments')->orderBy('id')->take(10)->get();

foreach ($posts as $post)
{
    echo $post->comments;
}

This example will create 2 queries: 1 to get all posts and another to get the posts' comments.

select * from posts

select * from comments where post_id in (1, 2, 3, 4, 5, ...)