Mapping Id to Model for controller API - asp.net-core

Question

I'm using asp.net core on a project. (I'm fairly new to it)
I have a User Model. the code below is a simplified version:
public class User
{
public int id { get; set; }
// attribute declaration
public ICollection<User> friends { get; set; }
}
I'm using automapper service to map my api to this Model:
public class UserResource
{
public UserResource()
{
this.friendsId = new List<int>();
}
public int id { get; set; }
// attribute declaration
public ICollection<int> friendsId { get; set; }
}
consider a post request to UserController with the following body:
{
"id" : 1
"friendsId": [2,3,4],
}
I want to map integers in friendsId to id of each user in friends collection. but I can't figure out what to do. here's what I've got:
CreateMap<UserResource,User>()
.ForMember(u => u.friends,opt => opt.MapFrom(????);
is this the right approach? if so how should I implement it?
or should I change my database model to this:
public class User
{
public int id { get; set; }
// attribute declaration
public ICollection<int> friendsId { get; set; }
}
Thank you in advance.

Answer 1

You'll need to implement a custom value resolver. These can be injected into, so you can access things like your context inside:
public class FriendsResolver : IValueResolver<UserResource, User, ICollection<User>>
{
private readonly ApplicationDbContext _context;
public FriendsResolver(ApplicationDbContext context)
{
_context = context ?? throw new ArgumentNullException(nameof(context));
}
public ICollection<User> Resolve(UserResource source, User destination, ICollection<User> destMember, ResolutionContext context)
{
var existingFriendIds = destMember.Select(x => x.Id);
var newFriendIds = source.friendsId.Except(existingFriendIds);
var removedFriendIds = existingFriendIds.Except(source.Friends);
destMember.RemoveAll(x => removedFriendIds.Contains(x.Id);
destMember.AddRange(_context.Users.Where(x => newFriendIds.Contains(x.Id).ToList());
return destMember;
}
}
Not sure if that's going to actually work as-is, as I just threw it together here, but it should be enough to get your going. The general idea is that you inject whatever you need into the value resolver and then use that to create the actual stuff you need to return. In this case, that means querying your context for the User entities with those ids. Then, in your CreateMap:
.ForMember(dest => dest.friends, opts => opts.ResolveUsing<FriendsResolver>());
This only covers one side of the relationship, though, so if you need to map the other way, you may need a custom resolver for that path as well. Here, I don't think you actually do. You should be able to just get by with:
.ForMember(dest => dest.friendsId, opts => opts.MapFrom(src => src.friends.Select(x => x.Id));

Answer 2

This would help
CreateMap<UserResource,User>()
.ForMember(u => u.friends,opt => opt.MapFrom(t => new User {FriendsId = t.friendsId);
public class User
{
...
public ICollection<User> friends { get; set; }
}
Where friends is ICollection<User> whereas UserResource class has ICollection<int>. There is type mismatch here. You need to map ICollection to ICollection that is why I casted new User ...