doctrine2/manual/codes/Hierarchical data - Nested set - Tree options.php

<?php
$options = array('has_many_roots' => true,			// enable many roots
	  			 'root_column_name' => 'root_id');	// set root column name, defaults to 'root_id'

// To create new root nodes, if you have manually set the root_id, then it will be used
// otherwise it will automatically use the next available root id
$root = new Menu();
$root->set('name', 'root');

// insert first root, will auto be assigned root_id = 1
$manager->getTable('Menu')->getTree()->createRoot($root);

$another_root = new Menu();
$another_root->set('name', 'another root');

// insert another root, will auto be assigned root_id = 2
$manager->getTable('Menu')->getTree()->createRoot($another_root);

// fetching a specifc root
$root = $manager->getTable('Menu')->getTree()->fetchRoot(1);
$another_root = $manager->getTable('Menu')->getTree()->fetchRoot(2);

// fetching all roots
$roots = $manager->getTable('Menu')->getTree()->fetchRoots();
?>