<?php
class Post extends Model {
    public function user() {
        return $this->belongs_to('User');
    }
}

class User extends Model {
    public function posts() {
        return $this->has_many('Post'); // Note we use the model name literally - not a pluralised version
    }
}

// Select a particular user from the database
$user = Model::factory('User')->find_one($user_id);
// Find the posts associated with the user
$posts = $user->posts()->find_many();

// Select a particular post from the database
$post = Model::factory('Post')->find_one($post_id);
// Find the user associated with the post
$user = $post->user()->find_one();

$posts = Model::factory('Post')->find_many();
foreach ($posts as $post) {
     echo($post->user()->find_one()->username); // generates a query each iteration
}