February 1, 2021

Eager loading hasMany relationship in Laravel

Eager loading relationships in Laravel makes it really easy to avoid N+1 query problems. This occurs when you run an additional query for each result in a previous query. Take for example a model Post that has a hasOne relationship with User.

$posts = Post::all();

foreach ($posts as $post) {
    // Retrieve the post author
    echo $post->user->name;

This would run an additional query for each post author. To avoid this we use eager loading. This essentially lazy loads the relationship into one extra query.

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

Now imagine the reverse of that example. You want to query all users and get their latest post. Easy we use eager loading to lazy load the posts relationship.

$users = User::with('posts')->get();

In this case eager loading causes to load all posts for every user. This is bad because we only need the latest post.

Because the user model has a hasMany relationship with posts, all posts are eager loaded for every user. To solve this you might think a simple ->limit(1) would do the trick. But that does not work.

The solution is actually pretty simple. In order to load the latest post you can simply create a new hasOne relationship on the User model.

public function latestPost()
    return $this->hasOne(Post::class)->latest();

When you use this relationship with eager loading only one post will be queried per user.

$users = User::with('latestPost')->get();

foreach ($users as $user) {
    // Retrieve the latest post
    echo $user->latestPost->title;

If you're wondering how you can easily keep track of how many queries are run I highly recommend you install barryvdh/laravel-debugbar.