withAggregate method

Under the hood, the withAvg/withCount/withSum and other methods in Eloquent use the 'withAggregate' method. You can use this method to add a subselect based on a relationship

 1// Eloquent Model
 2class Post extends Model
 3{
 4    public function user()
 5    {
 6        return $this->belongsTo(User::class);
 7    }
 8}
 9 
10// Instead of eager loading all users...
11$posts = Post::with('user')->get();
12 
13// You can add a subselect to only retrieve the user's name...
14$posts = Post::withAggregate('user', 'name')->get();
15 
16// This will add a 'user_name' attribute to the Post instance:
17$posts->first()->user_name;

Tip given by @pascalbaljet