title: Map multiple fields to one with MapStruct
body: <p>I have these 3 classes in separate files</p>
  
  <pre><code>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;
  
  }
  </code></pre>
  
  <p>I have the following mapper</p>
  
  <pre><code>@Mapper
  public interface BookMapper { 
  
      BookMapper INSTANCE = Mappers.getMapper(BookMapper.class);
  
      @Mappings({
              @Mapping(source = "author.lastName", target = "author")
      })
      BookDTO toDTO(Book book);
  
  }
  </code></pre>
  
  <p>this currently only maps the lastName and works, and I want to map the author string in Book with </p>
  
  <pre><code>author.firstName + " " + author.lastName
  </code></pre>
  
  <p>how could I do that? I have not been able to find anything in the MapStruct Documentation.</p>