AutoMapper problem with Dropdown

I´m started to work with AutoMapper today... But I´m having some problem with Dropdown model...

What I have so far :

User Model

public class User : Entity
{          
    public virtual string Name { get; set; }   
    public virtual string Email { get; set; }
    public virtual string Password { get; set; } 
    public virtual Role Role { get; set; }      
}

Role Model

public class Role : Entity
{          
    public virtual string Name { get; set; }   
}

UserUpdateViewModel

public class UserUpdateViewModel
{
    public int Id{get;set;}

    [Required(ErrorMessage = "Required.")]
    public virtual string Name { get; set; }

    [Required(ErrorMessage = "Required."), Email(ErrorMessage = "Email Invalid."), Remote("EmailExists", "User", ErrorMessage = "Email already in use.")]
    public virtual string Email { get; set; }

    [Required(ErrorMessage = "Required.")]
    public virtual string Password { get; set; }

    [Required(ErrorMessage = "Required")]
    public virtual string ConfirmPassword { get; set; }

    [Required(ErrorMessage = "Required.")]
    public int RoleId { get; set; }

    public IList<Role> Roles { get; set; }
}

UserController

public ActionResult Update(int id=-1)
{
      var _user = (_userRepository.Get(id));
      if (_user == null)
          return RedirectToAction("Index");

       Mapper.CreateMap<User, UserUpdateViewModel>();
       var viewModel = Mapper.Map<User, UserUpdateViewModel>(_user);

       viewModel.Roles = _roleRepository.GetAll();

       return View(viewModel);
}

[HttpPost, Transaction]
public ActionResult Update(UserViewModel user)
{
     if (ModelState.IsValid)
     {
         user.Password = _userService.GetPasswordHash(user.Password);

         Mapper.CreateMap<UserViewModel, User>();
         var model = Mapper.Map<UserViewModel, User>(user);  //model.Role = null

         _userRepository.SaveOrUpdate(model); //ERROR, because model.Role = null
         return Content("Ok");
     }
    开发者_JAVA百科 return Content("Erro").         
}

View Update

...
@Html.DropDownListFor(model => model.RoleId, new SelectList(Model.Roles, "Id", "Name"), "-- Select--", new { @class = "form radius" })   
...

Some considerations:

1 - I´m returning Content() because is all Ajax enabled using HTML 5 PushState etc etc

2 - In my Update(POST one) method, my model returned by Autommapper has Role = null

Why my Role returned by Automapper is null?

Is that the right way to work with AutoMapper? Any tip?

Thanks

---

The map is failing because you are trying to map a single Role directly to a collection of Roles. And a collection of Roles back to a single Role. You cant directly map between these as they are different types.

If you wanted to map a Role to a List then you could use a custom value resolver.

    Mapper.CreateMap<User , UserUpdateViewModel>()
    .ForMember(dest => dest.Roles, opt => opt.ResolveUsing<RoleToCollectionResolver>())


    Public class RoleToCollectionResolver: ValueResolver<User,IList<Role>>{ 
        Protected override IList<Role> ResolveCore(User source){
            var roleList = new List<Role>();
            roleList.Add(source.Role);
            Return roleList;
    }
}