Laravel基础数据库with函数技术咨询及致谢

Hey there! Let's break down Laravel's with() function—total game-changer for handling database relationships without those annoying N+1 query issues. I'll walk you through the basics, use cases, and some pro tips to level up your Eloquent skills.

What is with() actually for? #

At its core, with() enables eager loading—a way to load all necessary related model data in advance, instead of triggering new database queries every time you access a relationship.

Let's use a classic example to illustrate the problem it solves:
Suppose you have a Post model that hasMany Comment models. Without eager loading:

$posts = Post::all();

foreach ($posts as $post) {
    // Each loop triggers a new SELECT query for comments on this post
    echo "Post has " . $post->comments->count() . " comments";
}

This results in 1 + N queries (1 for all posts, N for each post's comments). With with(), we fix this:

$posts = Post::with('comments')->get();

foreach ($posts as $post) {
    // No extra queries—comments are already loaded!
    echo "Post has " . $post->comments->count() . " comments";
}

Now we only run 2 queries total: one for posts, one for all comments linked to those posts.

Loading Multiple Relationships at Once #

You can load multiple unrelated relationships in a single with() call by passing an array of relationship names:

// Load posts with their author (belongsTo User) and category (belongsTo Category)
$posts = Post::with('author', 'category')->get();

Loading Nested Relationships #

If your relationships have their own relationships (e.g., a Comment belongs to a User), use dot notation to load nested data:

// Load posts, their comments, and each comment's author
$posts = Post::with('comments.author')->get();

Constraining Eager Loads #

You don't always need every related record. Use a closure to filter or modify the eager load query:

// Only load published comments for each post
$posts = Post::with(['comments' => function ($query) {
    $query->where('is_published', true)->orderBy('created_at', 'desc');
}])->get();

You can also load only specific fields from related models (just remember to include the foreign key so Laravel can match records):

// Load only the author's id, name, and email (include user_id, the foreign key)
$posts = Post::with(['author:id,name,email,user_id'])->get();

Lazy Eager Loading #

If you already fetched a model collection and later decide you need related data, use load() instead of with():

$posts = Post::all();

// Later in your code, load comments for the existing posts
$posts->load('comments');

Use loadMissing() to only load relationships that haven't been loaded yet:

// Load comments only if they aren't already present
$posts->loadMissing('comments');

Bonus: withCount() for Relationship Counts #

If you just need the number of related records (not the records themselves), use withCount() to avoid loading unnecessary data:

// Add a `comments_count` attribute to each post
$posts = Post::withCount('comments')->get();

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

You can even add constraints to the count:

// Count only published comments, and name the attribute `published_comments_count`
$posts = Post::withCount([
    'published_comments' => function ($query) {
        $query->where('is_published', true);
    }
])->get();

内容的提问来源于stack exchange，提问作者NewbieCoder