How do I model a tree/hierarchy?

From PHP on Trax

Tree structures or hierarchies are simple to design in PoT. Simply add the parent_id column to the table and use something like this in your model:

<?
class Category extends ActiveRecord {
    public $belongs_to = array( "parent" => array(
                                            "class_name"  => "Category")
                               );
    public $has_many   = array( "children" => array(
                                             "foreign_key" => "parent_id",
                                             "class_name"  => "Category")
                               );  
}
?>

In the controller we only load entries that have a parent_id of 0 (root elements):

<?
class CategoryController extends ApplicationController {
    function tree() {
        $category = new Category();
        $this->categories = $category->find_all("parent_id=0");
    }
}
?>

And finally we could use a function like this to show a structured list of the tree:

<?php
function cattree($items) {
    $r = "";
    if (count($items) && is_array($items)) {
        $r .= "<ul>\n";
        foreach ($items as $item) {
            $r .= '<li>' . $item->title . ' [' . $item->id . '|' . $item->parent_id . ']';
            if (count($item->children)) {
                $r .= cattree($item->children);    
            }
            $r .= "</li>\n";    
        }
        $r .= "</ul>\n";
    }
    return $r;
}
?>
<?= cattree($categories); ?>

• Category 1 [1|0]
  • Category 4 [4|1]
  • Category 5 [5|1]
    • Category 7 [7|5]
• Category 2 [2|0]
  • Category 6 [6|2]
• Category 3 [3|0]