Laravel (Eloquent) equivalent of Mongoose's populate() method
If you have defined a posts() relationship in your User model, when retrieving the users, you can eager load the related data:
$users = User::with('posts')->get();
or a specific User:
$user = User::with('posts')->find($id);
Then $user->posts
would contain a collection of the related Post models.
I suggest you read through https://laravel.com/docs/5.5/eloquent-relationships