public function setRel($relations = null)
        {
            $this->_rel = array();

            if (null !== $relations) {

                if (!is_array($relations)) {
                    throw new Exception(
                            'Invalid argument: $relations must be an array');
                }

                foreach ($relations as $name => $relation) {
                    if (is_string($name)) {
                        $this->_rel[$name] = $relation;
                    }
                }
            }

            return $this;
        }