Tree

It’s fairly common to want to store hierarchical data in a database table. Examples of such data might be categories with unlimited subcategories, data related to a multilevel menu system or a literal representation of hierarchy such as is used to store access control objects with ACL logic.

For small trees of data, or where the data is only a few levels deep it is simple to add a parent_id field to your database table and use this to keep track of which item is the parent of what. Bundled with cake however, is a powerful behavior which allows you to use the benefits of MPTT logic without worrying about any of the intricacies of the technique - unless you want to ;).

Requirements

To use the tree behavior, your database table needs 3 fields as listed below (all are ints):

  • parent - default fieldname is parent_id, to store the id of the parent object
  • left - default fieldname is lft, to store the left value of the current row.
  • right - default fieldname is rght, to store the right value of the current row.

If you are familiar with MPTT logic you may wonder why a parent field exists - quite simply it’s easier to do certain tasks if a direct parent link is stored on the database - such as finding direct children.

Basic Usage

The tree behavior has a lot packed into it, but let’s start with a simple example - create the following database table and put some data in it:

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 the purpose of checking that everything is setup correctly, we can create a test method and output the contents of our category tree to see what it looks like. With a simple controller:

<?php
class CategoriesController extends AppController {

        var $name = 'Categories';

        function index() {
                $this->data = $this->Category->generatetreelist(null, null, null, '&nbsp;&nbsp;&nbsp;');
                debug ($this->data); die;
        }
}
?>

and an even simpler model definition:

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

We can check what our category tree data looks like by visiting /categories You should see something like this:

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

Adding data

In the previous section, we used existing data and checked that it looked hierarchal via the method generatetreelist. However, usually you would add your data in exactly the same way as you would for any model. For example:

// pseudo controller code
$data['Category']['parent_id'] =  3;
$data['Category']['name'] =  'Skating';
$this->Category->save($data);

When using the tree behavior its not necessary to do any more than set the parent_id, and the tree behavior will take care of the rest. If you don’t set the parent_id, the tree behavior will add to the tree making your new addition a new top level entry:

// pseudo controller code
$data = array();
$data['Category']['name'] =  'Other People\'s Categories';
$this->Category->save($data);

Running the above two code snippets would alter your tree as follows:

  • My Categories
    • Fun
      • Sport
        • Surfing
        • Extreme knitting
        • Skating New
      • Friends
        • Gerald
        • Gwendolyn
    • Work
      • Reports
        • Annual
        • Status
      • Trips
        • National
        • International
  • Other People’s Categories New

Modifying data

Modifying data is as transparent as adding new data. If you modify something, but do not change the parent_id field - the structure of your data will remain unchanged. For example:

// pseudo controller code
$this->Category->id = 5; // id of Extreme knitting
$this->Category->save(array('name' =>'Extreme fishing'));

The above code did not affect the parent_id field - even if the parent_id is included in the data that is passed to save if the value doesn’t change, neither does the data structure. Therefore the tree of data would now look like:

  • My Categories
    • Fun
      • Sport
        • Surfing
        • Extreme fishing Updated
        • Skating
      • Friends
        • Gerald
        • Gwendolyn
    • Work
      • Reports
        • Annual
        • Status
      • Trips
        • National
        • International
  • Other People’s Categories

Moving data around in your tree is also a simple affair. Let’s say that Extreme fishing does not belong under Sport, but instead should be located under Other People’s Categories. With the following code:

// pseudo controller code
$this->Category->id = 5; // id of Extreme fishing
$newParentId = $this->Category->field('id', array('name' => 'Other People\'s Categories'));
$this->Category->save(array('parent_id' => $newParentId));

As would be expected the structure would be modified to:

  • My Categories
    • Fun
      • Sport
        • Surfing
        • Skating
      • Friends
        • Gerald
        • Gwendolyn
    • Work
      • Reports
        • Annual
        • Status
      • Trips
        • National
        • International
  • Other People’s Categories
    • Extreme fishing Moved

Deleting data

The tree behavior provides a number of ways to manage deleting data. To start with the simplest example; let’s say that the reports category is no longer useful. To remove it and any children it may have just call delete as you would for any model. For example with the following code:

// pseudo controller code
$this->Category->id = 10;
$this->Category->delete();

The category tree would be modified as follows:

  • My Categories
    • Fun
      • Sport
        • Surfing
        • Skating
      • Friends
        • Gerald
        • Gwendolyn
    • Work
      • Trips
        • National
        • International
  • Other People’s Categories
    • Extreme fishing

Querying and using your data

Using and manipulating hierarchical data can be a tricky business. In addition to the core find methods, with the tree behavior there are a few more tree-orientated permutations at your disposal.

Most tree behavior methods return and rely on data being sorted by the lft field. If you call find() and do not order by lft, or call a tree behavior method and pass a sort order, you may get undesirable results.

Children

The children method takes the primary key value (the id) of a row and returns the children, by default in the order they appear in the tree. The second optional parameter defines whether or not only direct children should be returned. Using the example data from the previous section:

$allChildren = $this->Category->children(1); // a flat array with 11 items
// -- or --
$this->Category->id = 1;
$allChildren = $this->Category->children(); // a flat array with 11 items

// Only return direct children
$directChildren = $this->Category->children(1, true); // a flat array with 2 items

If you want a recursive array use find('threaded')

Counting children

As with the method children, childCount takes the primary key value (the id) of a row and returns how many children it has. The second optional parameter defines whether or not only direct children are counted. Using the example data from the previous section:

$totalChildren = $this->Category->childCount(1); // will output 11
// -- or --
$this->Category->id = 1;
$directChildren = $this->Category->childCount(); // will output 11

// Only counts the direct descendants of this category
$numChildren = $this->Category->childCount(1, true); // will output 2

generatetreelist

generatetreelist ($conditions=null, $keyPath=null, $valuePath=null, $spacer= '_', $recursive=null)

This method will return data similar to find(‘list’), with an indented prefix to show the structure of your data. Below is an example of what you can expect this method to return see the api for the other find-like parameters.

array(
    [1] =>  "My Categories",
    [2] =>  "_Fun",
    [3] =>  "__Sport",
    [4] =>  "___Surfing",
    [16] => "___Skating",
    [6] =>  "__Friends",
    [7] =>  "___Gerald",
    [8] =>  "___Gwendolyn",
    [9] =>  "_Work",
    [13] => "__Trips",
    [14] => "___National",
    [15] => "___International",
    [17] => "Other People's Categories",
    [5] =>  "_Extreme fishing"
)

getparentnode

This convenience function will, as the name suggests, return the parent node for any node, or false if the node has no parent (its the root node). For example:

$parent = $this->Category->getparentnode(2); //<- id for fun
// $parent contains All categories

getpath

The ‘path’ when refering to hierachial data is how you get from where you are to the top. So for example the path from the category “International” is:

  • My Categories
    • ...
    • Work
      • Trips
        • ...
        • International

Using the id of “International” getpath will return each of the parents in turn (starting from the top).

$parents = $this->Category->getpath(15);
// contents of $parents
array(
    [0] =>  array('Category' => array('id' => 1, 'name' => 'My Categories', ..)),
    [1] =>  array('Category' => array('id' => 9, 'name' => 'Work', ..)),
    [2] =>  array('Category' => array('id' => 13, 'name' => 'Trips', ..)),
    [3] =>  array('Category' => array('id' => 15, 'name' => 'International', ..)),
)

Advanced Usage

The tree behavior doesn’t only work in the background, there are a number of specific methods defined in the behavior to cater for all your hierarchical data needs, and any unexpected problems that might arise in the process.

moveDown

Used to move a single node down the tree. You need to provide the ID of the element to be moved and a positive number of how many positions the node should be moved down. All child nodes for the specified node will also be moved.

Here is an example of a controller action (in a controller named Categories) that moves a specified node down the tree:

function movedown($name = null, $delta = null) {
        $cat = $this->Category->findByName($name);
        if (empty($cat)) {
            $this->Session->setFlash('There is no category named ' . $name);
            $this->redirect(array('action' => 'index'), null, true);
        }

        $this->Category->id = $cat['Category']['id'];

        if ($delta > 0) {
            $this->Category->moveDown($this->Category->id, abs($delta));
        } else {
            $this->Session->setFlash('Please provide the number of positions the field should be moved down.');
        }

        $this->redirect(array('action' => 'index'), null, true);
    }

For example, if you’d like to move the “Sport” category one position down, you would request: /categories/movedown/Sport/1.

moveUp

Used to move a single node up the tree. You need to provide the ID of the element to be moved and a positive number of how many positions the node should be moved up. All child nodes will also be moved.

Here’s an example of a controller action (in a controller named Categories) that moves a node up the tree:

function moveup($name = null, $delta = null){
        $cat = $this->Category->findByName($name);
        if (empty($cat)) {
            $this->Session->setFlash('There is no category named ' . $name);
            $this->redirect(array('action' => 'index'), null, true);
        }

        $this->Category->id = $cat['Category']['id'];

        if ($delta > 0) {
            $this->Category->moveup($this->Category->id, abs($delta));
        } else {
            $this->Session->setFlash('Please provide a number of positions the category should be moved up.');
        }

        $this->redirect(array('action' => 'index'), null, true);

    }

For example, if you would like to move the category “Gwendolyn” up one position you would request /categories/moveup/Gwendolyn/1. Now the order of Friends will be Gwendolyn, Gerald.

removeFromTree

removeFromTree($id=null, $delete=false)

Using this method wil either delete or move a node but retain its sub-tree, which will be reparented one level higher. It offers more control than ``delete()Models, which for a model using the tree behavior will remove the specified node and all of its children.

Taking the following tree as a starting point:

  • My Categories
    • Fun
      • Sport
        • Surfing
        • Extreme knitting
        • Skating

Running the following code with the id for ‘Sport’

$this->Node->removeFromTree($id);

The Sport node will be become a top level node:

  • My Categories
    • Fun
      • Surfing
      • Extreme knitting
      • Skating
  • Sport Moved

This demonstrates the default behavior of removeFromTree of moving the node to have no parent, and re-parenting all children.

If however the following code snippet was used with the id for ‘Sport’

$this->Node->removeFromTree($id,true);

The tree would become

  • My Categories
    • Fun
      • Surfing
      • Extreme knitting
      • Skating

This demonstrates the alternate use for removeFromTree, the children have been reparented and ‘Sport’ has been deleted.

reorder

This method can be used to sort hierarchical data.

Data Integrity

Due to the nature of complex self referential data structures such as trees and linked lists, they can occasionally become broken by a careless call. Take heart, for all is not lost! The Tree Behavior contains several previously undocumented features designed to recover from such situations.

These functions that may save you some time are:

recover(&$model, $mode = ‘parent’, $missingParentAction = null)

The mode parameter is used to specify the source of info that is valid/correct. The opposite source of data will be populated based upon that source of info. E.g. if the MPTT fields are corrupt or empty, with the $mode ‘parent’ the values of the parent_id field will be used to populate the left and right fields. The missingParentAction parameter only applies to “parent” mode and determines what to do if the parent field contains an id that is not present.

reorder(&$model, $options = array())

Reorders the nodes (and child nodes) of the tree according to the field and direction specified in the parameters. This method does not change the parent of any node.

The options array contains the values ‘id’ => null, ‘field’ => $model->displayField, ‘order’ => ‘ASC’, and ‘verify’ => true, by default.

verify(&$model)

Returns true if the tree is valid otherwise an array of (type, incorrect left/right index, message).