1. The database
2. The view
3. The controller

The database ¶

This example uses a MySQL DB with a table named tree with the fields id, name, and parent_id. The parent_id will be NULL for root elements. The SQL is kept simple (no autoinc, no FK, etc).

[sql]
CREATE TABLE tree (
  id INTEGER UNSIGNED NOT NULL,
  name VARCHAR(50) NOT NULL,
  parent_id INTEGER UNSIGNED,
  PRIMARY KEY (id)
)

MySQL has no recursive queries. In order to fetch tree nodes, you have to recursively send SQL queries for each node, asking for its children. The easiest way to do so is to send these queries with AJAX, so that the tree can be displayed even if the deep nodes haven't been fetched yet. If you're using Oracle or Postgresql, there are other solutions, though this will also work.

The view ¶

In your view, add:

~ `php <?php $this->widget(

'CTreeView',
array('url' => array('ajaxFillTree'))

); ?> `~

This will create a tree widget CTreeView on your page, and its data will be fetched using the ajaxFillTree of the current controller.

The controller ¶

So we have to add this action to the controller:

~ `php /**

 * Fills the JS tree on an AJAX request.
 * Should receive parent node ID in $_GET['root'],
 *  with 'source' when there is no parent.
 */
public function actionAjaxFillTree()
{
	// accept only AJAX request (comment this when debugging)
	if (!Yii::app()->request->isAjaxRequest) {
		exit();
	}
	// parse the user input
	$parentId = "NULL";
	if (isset($_GET['root']) && $_GET['root'] !== 'source') {
		$parentId = (int) $_GET['root'];
	}
	// read the data (this could be in a model)
	$children = Yii::app()->db->createCommand(
		"SELECT m1.id, m1.name AS text, m2.id IS NOT NULL AS hasChildren "
		. "FROM tree AS m1 LEFT JOIN tree AS m2 ON m1.id=m2.parent_id "
		. "WHERE m1.parent_id <=> $parentId "
		. "GROUP BY m1.id ORDER BY m1.name ASC"
	)->queryAll();
	echo str_replace(
		'"hasChildren":"0"',
		'"hasChildren":false',
		CTreeView::saveDataAsJson($children)
	);
}

When the page loads, an AJAX request will be sent to fill the first level of the tree. It'll have the GET parameter `root` set to `source` (this is the behavior of the [Treeview](http://bassistance.de/jquery-plugins/jquery-plugin-treeview/) JS plugin that [CTreeView](http://www.yiiframework.com/doc/api/1.1/CTreeView/) uses). We suppose here that the corresponding nodes (the root nodes) have a `parent_id` set to `NULL`. In SQL, "=" can't compare a value with `NULL`, so we have to use the `<=>` operator instead.

The other AJAX requests will have an integer value (the parent node's id) in `$_GET['root']`. We typecast this to "int" for security.

Then the code reads the data in the database. We need a *LEFT JOIN* to find if a node has children. If each row already has a `hasChildren` field, you can remove this join and your SQL will be faster.

When sending the JSON-encoded result, there's a little trick: the javascript wants the `hasChildren` attribute to be a boolean. But the SQL result just contains 0 or 1, so we have to convert it, at least for the "false" case. Instead of operating on the string result, one could modify the PHP array with a `foreach`.

### If AJAX fails

If your AJAX query fails, you should try to debug it in your browser. Firefox, Chrome and Opera have good developer toolbars that will show you the AJAX requests sent and what their answers were. You may find that some HTML is appended at the end of the expected JSON answer. I had this problem with Yii'JS logs and a toolbar extension. In this case, insert an `exit();` at the end of the action. Or even better: `Yii::logger->flush(); exit();`.

### Final note

This is meant as a simple introduction. There are many ways to enhance this:

* Refactor the controller action by putting its 2 last lines into a model.
* Increase the abstraction level, so that the code can be used on various models, as long as they share the same table structure.
* If the tree content rarely changes, you may consider using a different structure, like nested sets. This will make most queries easier, e.g. fetching the path of a node.