public function modified() {
		$fields = array_fill_keys(array_keys($this->_data), false);
		foreach ($this->_updated as $field => $value) {
			if (is_object($value) && method_exists($value, 'modified')) {
				if (!isset($this->_data[$field])) {
					$fields[$field] = true;
					continue;
				}
				$modified = $value->modified();
				$fields[$field] = $modified === true || is_array($modified) && in_array(true, $modified, true);
			} else {
				$fields[$field] = !isset($fields[$field]) || $this->_data[$field] !== $this->_updated[$field];
			}
		}
		return $fields;
	}