When you want to store a heirarchical data in the database table, then we use the tree structure in cakephp.

For e.g : When you need to form a heirarchy of categories & subcategories like:-

My Categories
  Fun
     Sport
       Surfing
       Extreme knitting
      
     Friends
        Gerald
        Gwendolyn
   Work
     Reports
        Annual
         Status
      Trips
       National
       International

For creating tree structure in cakephp, we need to create 3 columns in the database table namely, 

1. Parent -> Default field name is parent_id, which stores the id of the parent object.
2. Left-> Default fieldname is lft, to store left value of current row.
3. Right-> Default fieldname is rght, to store the right value of the current row.

In database, create the following table and insert data in that table ;-

CREATE TABLE categories (
    id INTEGER(10) UNSIGNED NOT NULL AUTO_INCREMENT,
    parent_id INTEGER(10) DEFAULT NULL,
    lft INTEGER(10) DEFAULT NULL,
    rght INTEGER(10) DEFAULT NULL,
    name VARCHAR(255) DEFAULT '',
    PRIMARY KEY  (id)
);

INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (1, 'My Categories', NULL, 1, 30);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (2, 'Fun', 1, 2, 15);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (3, 'Sport', 2, 3, 8);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (4, 'Surfing', 3, 4, 5);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (5, 'Extreme knitting', 3, 6, 7);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (6, 'Friends', 2, 9, 14);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (7, 'Gerald', 6, 10, 11);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (8, 'Gwendolyn', 6, 12, 13);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (9, 'Work', 1, 16, 29);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (10, 'Reports', 9, 17, 22);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (11, 'Annual', 10, 18, 19);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (12, 'Status', 10, 20, 21);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (13, 'Trips', 9, 23, 28);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (14, 'National', 13, 24, 25);
INSERT INTO
  `categories` (`id`, `name`, `parent_id`, `lft`, `rght`)
VALUES
  (15, 'International', 13, 26, 27);

For creating tree structure, we have to make two views namely, add.ctp & index.ctp.

In add.ctp, add the following code :-

<?php

echo $this->Html->link('Back',array('action'=>'index', 'method'=>'post'));

echo $this->Form->create('Category',array('controller' => 'categories'));
 
echo $this->Form->input('parent_id',array( 'label'=>'Parent'));

echo $this->Form->end('Add');

?>

In index.ctp, add the following code :-

<?php

echo $this->Html->link('Add Category',array('action'=>'add'));

echo '<table>';

foreach($userlist as $key=>$value){
	echo '<tr><td>';
		echo $value;
	echo '</td></tr>';
}
echo '</table>';
?>

In Controller.php, add the following code:-

<?php
class UsersController extends AppController {

 var $uses = array('User', 'Category');

 function index() {
    $data = $this->Category->generateTreeList(null, null, null, '&nbsp;&nbsp;&nbsp;');
        //print_r($data);die;
    $this->set('userlist',$data);
}


function add() {
    if (!empty($this->data)) {
        if($this->request->post)
        $this->Category->save($this->data);
        $this->redirect(array('action'=>'index'));
    } else {
        $parents[0] = "[ --Select Category-- ]";
        $categorylist = $this->Category->generateTreeList(null,null,null,"");
        if($categorylist) {
            foreach ($categorylist as $key=>$value)
                $parents[$key] = $value;
        }
        $this->set(compact('parents'));
    }
}



}


?>

In model.php, add the following code :-

<?php
	
	class Category extends AppModel {
		var $name = 'Category';
		var $actsAs = array('Tree');
	}
?>