Tree

class Cake\ORM\Behavior\TreeBehavior

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 departments in a company.

Relational databases are usually not well suited for storing and retrieving this type of data, but there are a few known techniques that can make them effective for working with multi-level information.

The TreeBehavior helps you maintain a hierarchical data structure in the database that can be queried without much overhead and helps reconstruct the tree data for finding and displaying processes.

Requirements

This behavior requires the following columns in your table:

  • parent_id (nullable) The column holding the ID of the parent row
  • lft (integer, signed) Used to maintain the tree structure
  • rght (integer, signed) Used to maintain the tree structure

You can configure the name of those fields should you need to customize them. More information on the meaning of the fields and how they are used can be found in this article describing the MPTT logic

Warning

The TreeBehavior does not support composite primary keys at this point in time.

A Quick Tour

You enable the Tree behavior by adding it to the Table you want to store hierarchical data in:

class CategoriesTable extends Table
{
    public function initialize(array $config)
    {
        $this->addBehavior('Tree');
    }
}

Once added, you can let CakePHP build the internal structure if the table is already holding some rows:

$categories = TableRegistry::get('Categories');
$categories->recover();

You can verify it works by getting any row from the table and asking for the count of descendants it has:

$node = $categories->get(1);
echo $categories->childCount($node);

Getting a flat list of the descendants for a node is equally easy:

$descendants = $categories->find('children', ['for' => 1]);

foreach ($descendants as $category) {
    echo $category->name . "\n";
}

If you instead need a threaded list, where children for each node are nested in a hierarchy, you can stack the ‘threaded’ finder:

$children = $categories
    ->find('children', ['for' => 1])
    ->find('threaded')
    ->toArray();

foreach ($children as $child) {
    echo "{$child->name} has " . count($child->children) . " direct children";
}

Traversing threaded results usually requires recursive functions in, but if you only require a result set containing a single field from each level so you can display a list, in an HTML select for example, it is better to use the ‘treeList’ finder:

$list = $categories->find('treeList');

// In a CakePHP template file:
echo $this->Form->control('categories', ['options' => $list]);

// Or you can output it in plain text, for example in a CLI script
foreach ($list as $categoryName) {
    echo $categoryName . "\n";
}

The output will be similar to:

My Categories
_Fun
__Sport
___Surfing
___Skating
_Trips
__National
__International

The treeList finder takes a number of options:

  • keyPath: A dot separated path to fetch the field to use for the array key, or a closure to return the key out of the provided row.
  • valuePath: A dot separated path to fetch the field to use for the array value, or a closure to return the value out of the provided row.
  • spacer: A string to be used as prefix for denoting the depth in the tree for each item

An example of all options in use is:

$query = $categories->find('treeList', [
    'keyPath' => 'url',
    'valuePath' => 'id',
    'spacer' => ' '
]);

One very common task is to find the tree path from a particular node to the root of the tree. This is useful, for example, for adding the breadcrumbs list for a menu structure:

$nodeId = 5;
$crumbs = $categories->find('path', ['for' => $nodeId]);

foreach ($crumbs as $crumb) {
    echo $crumb->name . ' > ';
}

Trees constructed with the TreeBehavior cannot be sorted by any column other than lft, this is because the internal representation of the tree depends on this sorting. Luckily, you can reorder the nodes inside the same level without having to change their parent:

$node = $categories->get(5);

// Move the node so it shows up one position up when listing children.
$categories->moveUp($node);

// Move the node to the top of the list inside the same level.
$categories->moveUp($node, true);

// Move the node to the bottom.
$categories->moveDown($node, true);

Configuration

If the default column names that are used by this behavior don’t match your own schema, you can provide aliases for them:

public function initialize(array $config)
{
    $this->addBehavior('Tree', [
        'parent' => 'ancestor_id', // Use this instead of parent_id
        'left' => 'tree_left', // Use this instead of lft
        'right' => 'tree_right' // Use this instead of rght
    ]);
}

Node Level (Depth)

Knowing the depth of tree nodes can be useful when you want to retrieve nodes only upto a certain level for e.g. when generating menus. You can use the level option to specify the field that will save level of each node:

$this->addBehavior('Tree', [
    'level' => 'level', // Defaults to null, i.e. no level saving
]);

If you don’t want to cache the level using a db field you can use TreeBehavior::getLevel() method to get level of a node.

Scoping and Multi Trees

Sometimes you want to persist more than one tree structure inside the same table, you can achieve that by using the ‘scope’ configuration. For example, in a locations table you may want to create one tree per country:

class LocationsTable extends Table
{

    public function initialize(array $config)
    {
        $this->addBehavior('Tree', [
            'scope' => ['country_name' => 'Brazil']
        ]);
    }

}

In the previous example, all tree operations will be scoped to only the rows having the column country_name set to ‘Brazil’. You can change the scoping on the fly by using the ‘config’ function:

$this->behaviors()->Tree->config('scope', ['country_name' => 'France']);

Optionally, you can have a finer grain control of the scope by passing a closure as the scope:

$this->behaviors()->Tree->config('scope', function ($query) {
    $country = $this->getConfigureContry(); // A made-up function
    return $query->where(['country_name' => $country]);
});

Recovering with custom sort field

New in version 3.0.14.

By default, recover() sorts the items using the primary key. This works great if this is a numeric (auto increment) column, but can lead to weird results if you use UUIDs.

If you need custom sorting for the recovery, you can set a custom order clause in your config:

$this->addBehavior('Tree', [
    'recoverOrder' => ['country_name' => 'DESC'],
]);

Saving Hierarchical Data

When using the Tree behavior, you usually don’t need to worry about the internal representation of the hierarchical structure. The positions where nodes are placed in the tree are deduced from the ‘parent_id’ column in each of your entities:

$aCategory = $categoriesTable->get(10);
$aCategory->parent_id = 5;
$categoriesTable->save($aCategory);

Providing inexistent parent ids when saving or attempting to create a loop in the tree (making a node child of itself) will throw an exception.

You can make a node a root in the tree by setting the ‘parent_id’ column to null:

$aCategory = $categoriesTable->get(10);
$aCategory->parent_id = null;
$categoriesTable->save($aCategory);

Children for the new root node will be preserved.

Deleting Nodes

Deleting a node and all its sub-tree (any children it may have at any depth in the tree) is trivial:

$aCategory = $categoriesTable->get(10);
$categoriesTable->delete($aCategory);

The TreeBehavior will take care of all internal deleting operations for you. It is also possible to only delete one node and re-assign all children to the immediately superior parent node in the tree:

$aCategory = $categoriesTable->get(10);
$categoriesTable->removeFromTree($aCategory);
$categoriesTable->delete($aCategory);

All children nodes will be kept and a new parent will be assigned to them.

The deletion of a node is based off of the lft and rght values of the entity. This is important to note when looping through the various children of a node for conditional deletes:

$descendants = $teams->find('children', ['for' => 1]);

foreach ($descendants as $descendant) {
    $team = $teams->get($descendant->id); // search for the up-to-date entity object
    if ($team->expired) {
        $teams->delete($team); // deletion reorders the lft and rght of database entries
    }
}

The TreeBehavior reorders the lft and rght values of records in the table when a node is deleted. As such, the lft and rght values of the entities inside $descendants (saved before the delete operation) will be inaccurate. Entities will have to be loaded and modified on the fly to prevent inconsistencies in the table.