protected static function _initRelationships()
    {
        $className = get_called_class();
        if (!empty(static::$_classRelationships[$className])) {
            $relationships = [];
            foreach (static::$_classRelationships[$className] as $relationship => $options) {
                if (is_array($options)) {
                    $relationships[$relationship] = static::_initRelationship($relationship, $options);
                }
            }
            static::$_classRelationships[$className] = $relationships;
        }
    }