My assumption going into this was that creating a tree-like relationship within one of my models would be fairly straightforward with Cake’s model relationships.  As it turns out, I was absolutely correct.

Let’s say we want to create a model called “Category”, and each category can belong to a parent category.  In other words, we want our application to recognize that every category may potentially belong to a higher-level category, and likewise, that every category may also have child categories beneath it.

Consider a Category model similar to the following:

CREATE TABLE `categories` (
  `id` INTEGER(12) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(64) NOT NULL,
  `parent_id` INTEGER(12) NULL,

  PRIMARY KEY(`id`)
);

The first step is to define our model (obviously):

<?php
class Category extends AppModel {
  var $name = 'Category';
}
?>

Next, let’s tell Cake that a category can belong to other categories using the belongsTo association:

< ?php

class Category extends AppModel {
  var $name = 'Category';

  var $belongsTo = array(
    'ParentCategory' => array(
      'className' => 'Category',
      'foreignKey' => 'parent_id'
  ));
}
?>

Finally, we want to tell Cake that a category can have other categories beneath it.  We’ll do this using the hasMany association:

< ?php

class Category extends AppModel {
  var $name = 'Category';

  var $belongsTo = array(
    'ParentCategory' => array(
      'className' => 'Category',
      'foreignKey' => 'parent_id'
  ));

  var $hasMany = array(
    'ChildCategory' => array(
      'className' => 'Category',
      'foreignKey' => 'parent_id'
  ));
}
?>

That’s it.  Create a controller for categories and turn on scaffolding, and you’ll see how nicely this all works out.