MapStruct add a new calculated field to the dto

Question

I'm trying to map an entity Order to a OrderDTO using MapStruct. I want to add to OrderDTO a new field total, this field is not available in the original entity Order and should be calculated using the information available in the Order (order entries price, quantity, taxes...). I created a new field total in the OrderDTO and I'm trying to map it by adding a default method to the mapper interface:

public interface OrderMapper {

    ...

    default BigDecimal orderToTotal(Order order){
        return logicToCalculateTotal();
    }
}

When I lunch the build MapStruct launch the error

Unmapped target property: "total".

Any idea how to solve this problem?

Thanks

Answer 1

There are multiple way to achieve what you need. The first way is to use @AfterMapping or @BeforeMapping. If you go with this your code will look like:

public interface OrderMapper {

    @Mapping(target = "total", ignore = true) // Needed so the warning does not shown, it is mapped in calculateTotal
    OrderDto map(Order order);

    @AfterMapping // or @BeforeMapping
    default void calculateTotal(Order order, @MappingTarget OrderDto dto) {
        dto.setTotal(logicToCalculateTotal());
    }
}

The alternative approach would be to do like you started, but you have to say that total is mapped from the Order.

Your mapper in the alternative approach would be:

public interface OrderMapper {

    @Mapping(target = "total", source = "order")// the source should be equal to the property name
    OrderDto map(Order order);

    default BigDecimal orderToTotal(Order order) {
        return logicToCalculateTotal();
    }
}