自动映射器多对多关系映射

Question

Let's say, in the BookDetails page (BookForDetailsDto) we also show the authors of that book (AuthorForListingDto). And moreover, I want to show this author list together with a little info (just the name and id) on the books (BookForAuthorListingDto) of each author.

I have a simple many-to-many relation consisting of Book, Author and BookAuthor objects.

public class Book {        
    public int Id { get; set; }
    public string Name { get; set; }
    public List<BookAuthor> Authors { get; set; }
}

public class Author {
    public int Id { get; set; }
    public string Name { get; set; }
    public List<BookAuthor> Books { get; set; }
}

public class BookAuthor {
    public int BookId { get; set; }
    public Book Book { get; set; }
    public int AuthorId { get; set; }
    public Author Author { get; set; }
}

And I have also 3 DTOs (where I am stoping an infinite loop):

public class BookForDetailsDto {
    public int Id { get; set; }
    public string Name { get; set; }
    public List<AuthorForListingDto> Authors { get; set; }
}

public class AuthorForListingDto {
    public int Id { get; set; }
    public string Name { get; set; }
    public List<BookForAuthorListingDto> Books { get; set; }
}

public class BookForAuthorListingDto {
    public int Id { get; set; }
    public string Name { get; set; }
}

Having a configuration as the following:

var config = new MapperConfiguration(cfg => {
    cfg.CreateMap<Book, BookForDetailsDto>();
    cfg.CreateMap<BookAuthor, AuthorForListingDto>();
    cfg.CreateMap<AuthorForListingDto, BookForAuthorListingDto>();
});

I'd like to perform a mapping from Book to BookForDetailsDto like this.

BookForDetailsDto BookDto = mapper.Map<BookForDetailsDto>(book);

But as a result, I get System.NullReferenceException. It seems like, just in the first level of mapping, AutoMapper cannot get Author information from BookAuthor object. I am searching for a configuration option but with no luck. I should say I am a newbie with automapper and if there is a simple solution I appreciate.

Note: I saw a comment which goes like "it is not a good practice to have reference in one DTO to second DTO". But I cannot figure out how to do otherwise, because ,for example, for a clickable/navigatable child_object we need at least "a key and a display_name", so a child object of type List seems inevitable.

Answer 1

A new day with a new head... I changed the mappings like the following and it works as expected:

        var config = new MapperConfiguration(cfg =>
        {
            cfg.CreateMap<Book, BookForDetailsDto>()
                .ForMember(dto => dto.Authorss, opt => opt.MapFrom(x => x.Authors.Select(a => a.Author)));
            cfg.CreateMap<BookAuthor, BookForAuthorListingDto >()
                .ForMember(res => res.Id, opt => opt.MapFrom(dto => dto.Book.Id))
                .ForMember(res => res.Name, opt => opt.MapFrom(dto => dto.Book.Name));
        });