public function getVersions($force = false)
    {
        if ($this->nb_project_version_list === null) {
            $this->nb_project_version_list = new CNabuProjectVersionList();
        }

        if ($this->nb_project_version_list->isEmpty() || $force) {
            $this->nb_project_version_list->clear();
            $this->nb_project_version_list->merge(CNabuProjectVersion::getAllProjectVersions($this));
        }

        return $this->nb_project_version_list;
    }