protected function tryToGetJoinColumnNameOfMappedBy($annotation){
        $annotation = $this->tryToGetJoinColumnAnnotationOfMappedBy($annotation);
        if ($annotation !== null) {
            if ($annotation instanceof JoinColumn) {
                return $annotation->name;
            } else if ($annotation instanceof JoinColumns) {
                if (count($annotation->value) > 1) {
                    throw new \Exception('Multiple foreign key is not supported');
                }
                return $annotation->value[0]->name;
            }
        }
        return '';
    }