Map multiple fields to one with MapStruct - mapstruct

Question

I have these 3 classes in separate files
public class Book {
#Id
#GeneratedValue
private Long id;
#NonNull
private String title;
#NonNull
private Author author;
}
public class Author {
#Id
#GeneratedValue
private Long id;
#NonNull
private String firstName;
#NonNull
private String lastName;
}
public class BookDTO {
private Long id;
#NonNull
private String title;
#NonNull
private String author;
}
I have the following mapper
#Mapper
public interface BookMapper {
BookMapper INSTANCE = Mappers.getMapper(BookMapper.class);
#Mappings({
#Mapping(source = "author.lastName", target = "author")
})
BookDTO toDTO(Book book);
}
this currently only maps the lastName and works, and I want to map the author string in Book with
author.firstName + " " + author.lastName
how could I do that? I have not been able to find anything in the MapStruct Documentation.

Answer 1

MapSruct does not support mapping multiple source properties into a single target property.
You have 2 ways to achieve this:
Using Mapping#expression
#Mapping( target = "author", expression = "java(book.getAuthor().getFirstName() + \" \" + book.getAuthor().getLastName())")
Using #AfterMapping or #BeforeMapping
#Mapper
public interface BookMapper {
BookMapper INSTANCE = Mappers.getMapper(BookMapper.class);
#Mapping(target = "author", ignore = true)
BookDTO toDTO(Book book);
#AfterMapping
default void setBookAuthor(#MappingTarget BookDTO bookDTO, Book book) {
Author author = book.getAuthor();
bookDTO.setAuthor(author.getFirstName() + " " + author.getLastName());
}
}