long	ConfigurationTable::key2id(const ConfigurationKey& key) {
    std::list<long>	ids = selectids(condition(key));
    if (ids.size() != 1) {
        std::string	msg = stringprintf("%s not found",
                                       key.toString().c_str());
        throw NoSuchEntry(msg);
    }
    return *ids.begin();
}