Handle potential columns in type conversion

[damian-kos]

How can I handle columns in type conversion which won't always be there?
For example when we add something by JOIN?
I know that I can v.get_number_of_columns() and work around this but this is not flexbile enough.

 // if another column which could be added by for example left join
    model.customer.phone = v.get<std::string>("phone"); // if it doesnt exists in basic query it will fail in type conversion

template <>
  struct type_conversion<Repair> {
    typedef values base_type;

    static void from_base(const values& v, indicator ind, Repair& model) {
      std::cout << "Converting from base for " << typeid(Repair).name() << std::endl;

      if (ind == i_null) {
        throw std::runtime_error("Null value fetched from database");
      }
      model.id = v.get<int>("id");
      model.customer.id = v.get<int>("customer_id");
      model.device.id = v.get<int>("model_id", -1);
      model.color.id = v.get<int>("color_id", -1);
      model.vis_note = v.get<std::string>("visible_desc");
      model.hid_note = v.get<std::string>("hidden_desc");
      model.price = v.get<double>("price");
      model.repair_state.id = v.get<int>("repair_state_id");
      model.sn_imei = v.get<std::string>("sn_imei");
      model.cust_device_id = v.get<int>("cust_device_id", -1);
     
     // if another column which could be added by for example left join
    model.customer.phone = v.get<std::string>("phone"); // if it doesnt exists in basic query it will fail in type conversion
    }

    static void to_base(const Repair& model, values& v, indicator& ind) {
      v.set("id", model.id);
      v.set("customer_id", model.customer.id);
      v.set("model_id", model.device.id, model.device.id == -1 ? i_null : i_ok);
      v.set("category_id", model.category.id, model.category.id == -1 ? i_null : i_ok);
      v.set("color_id", model.color.id, model.color.id == -1 ? i_null : i_ok);
      v.set("visible_desc", model.vis_note);
      v.set("hidden_desc", model.hid_note);
      v.set("price", model.price);
      v.set("repair_state_id", model.repair_state.id);
      v.set("sn_imei", model.sn_imei);
      v.set("cust_device_id", model.cust_device_id, model.cust_device_id == -1 ? i_null : i_ok);

      ind = i_ok;
    }
  };

// if another column which could be added by for example left join model.customer.phone = v.get<std::string>("phone"); // if it doesnt exists in basic query it will fail in type conversion