carjson

echo $json->toJSON( mixed $model, array $relations, array $attributes);

Yii::import('application.extensions.json.CArJSON');
$users = User::model()->findAll($criteria);
 
$json = new CArJSON();
// Define which relations do you want to load
$relations = array('posts', 'comments', 'votes');
// Define which attributes, foreach relations do you want to load
// root denote attributes for the main model
$attributes = array(
    'root' => array('id', 'username', 'email', 'first_name', 'last_name'),
        'posts' => array( 'id', 'author_id', 'content' ),
    'comments' => array( 'id', 'author_id', 'content' ),
    'votes' => null, // This will load all attributes
);
echo $json->toJSON($users, $relations, $attributes);