Problem with mapping objects with automapper

Question

I have a problem with mapping. Here is my model .

    public class Post
    {
        public int Id { get; set; }
        public string Description { get; set; }
        public DateTime Created { get; set; }
        public User User { get; set; }
        public int UserId { get; set; }
    }

My Dto:

public class PostForReturnDto
    {
        public PostForReturnDto()
        {
             Created = DateTime.Now;
        }
         public int Id { get; set; }
        public string Description { get; set; }
        public DateTime Created { get; set; }
        public string Author { get; set; }
    }

AutoMapperProfiles.cs

 CreateMap<Post, PostForReturnDto>()
            .ForMember(p => p.Author,
             opt => opt.MapFrom(src => src.User.KnownAs));

RepositoryContext

 public async Task<IEnumerable<Post>> GetPosts() {
             var posts = _context.Posts
            .Include(u => u.User)
            .OrderByDescending(p => p.Created)
            .ToListAsync();

            return await posts;
        }

While in debug mode in the PostController I receive the following information with the user object inside.

I'm trying to map KnownAs property from the User object to my DTO, but with no success. The error is:

AutoMapper.AutoMapperMappingException: Missing type map configuration or unsupported mapping. Mapping types: List 1 -> PostForReturnDto System.Collections.Generic.List 1[[DateApp.API.Models.Post, DateApp.API, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]] -> DateApp.API.Dtos.PostForReturnDto at lambda_method(Closure , List`1 , PostForReturnDto , ResolutionContext ) at lambda_method(Closure , Object , Object , ResolutionContext ) at AutoMapper.Mapper.Map[TDestination](Object source) in C:\\projects\\automapper\\src\\AutoMapper\\Mapper.cs:line 35

Answer 1

Seems that you are mapping the List<Post> not the Post instance like it should be

try like below

var posts = await _repo.GetPosts();

var res = posts.Select(_ => _mapper.Map<PostToReturn>(_));

return Ok(res);

or you can map the collection having generic argument as documentation says

IEnumerable<PostToReturn> postToReturn= mapper.Map<IEnumerable<Post>, IEnumerable<PostToReturn>>(posts);

UPD Seems your GetPosts() implementation lacks some Where or Take filter. It takes all posts right now and it can cause performance problem in the production if there are a lot of posts.