void Database::Add(const Date& date, const std::string& event){
    if (database_records_map.count(date) == 0) {
        database_records_map[date] = std::vector<std::string>(0);
    };
    auto location_in_event_vector = std::find(
        database_records_map[date].begin(),
        database_records_map[date].end(),
        event
    );
    if (location_in_event_vector == database_records_map[date].end()){
        database_records_map[date].push_back(event);
    };
    database_records_set.insert(
        make_pair(date, event)
    );
};