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

<code type="php">
//use these options in the setTableDefinition
$options = array('hasManyRoots' => true,			// enable many roots
	  			 'rootColumnName' => '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();
</code>