last error - database validation

app_model.php

<?php
class AppModel extends Model {

	public function retornaErro(){
		$db = ConnectionManager::getDataSource('default');
		return $db->lastError();
	}
}

usuarios_controller.php

<?php
	class UsuariosController extends AppController {

		public function add(){
			$usuario = array(
				'nome' => 'luiz'
			);

			if(!$this->Usuario->save($usuario)){
				print_r($this->Usuario->retornaErro());
			}
		}
	}