Mapping EF Entities to ViewModels maintaining ICollection and lazy loading

Question

This is something that has bugged me since the shift to EF 6. How do we now map collections through to view models such that mapping changes back are not painful using IEnumerables. Here is a code snippet below demonstrating my problem:

Entity - SS.Entity.Event

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<SS.Entity.User> Broadcasters { get; set; }
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
public virtual ICollection<SS.Entity.User> Viewers { get; set; }

Model - SS.Model.Event

public virtual ICollection<SS.Model.User> Broadcasters { get; set; }
public virtual ICollection<SS.Model.User> Viewers { get; set; }

Mapping Back to Entity After Modification of Collection

Broadcasters = e.Broadcasters.Select(u => new SS.Entity.User
{
    Id = u.Id,
    SkypeId = u.SkypeId,
    Name = u.Name
}).ToList(), // THIS IS THE PROBLEM
Viewers = e.Viewers.Select(u => new SS.Entity.User
{
    Id = u.Id,
    SkypeId = u.SkypeId,
    Name = u.Name
}).ToList() // THIS IS THE PROBLEM

The problem with this is I cannot map an ICollection to another ICollection as Select produces an IEnumerable which makes mapping back properties to EF afterwards a pain as I have to recreate the collection or enumerate it to update it. I know I'm missing something obvious, I have updated the ICollection's to be virtual as outlined in other answers but it is unclear to me how this helps.

Any help would be greatly appreciated!

Gerard

Answer 1

Assuming your ".ToList()" issue is while saving back to database, is this what your are looking for?:

 var event = new SS.Entity.Event {Name = "New Name" and other properties};

 IEnumerable<SS.Entity.User> broadcasters = e.Broadcasters
        .Select(u => new SS.Entity.User
             {
              Id = u.Id,
              SkypeId = u.SkypeId,
              Name = u.Name
           });

 var viewers = e.Viewers.Select(u =>
          new SS.Entity.User
          {
             Id = u.Id,
             SkypeId = u.SkypeId,
              Name = u.Name
          });

 //add broadcasters to event
 event.Broadcasters.AddRange(broadcasters);

 //add viewers to event
 event.Viewers.AddRange(viewers);

 dataContext.Events.Add(event);
 dataContext.SaveChanges();

Answer 2

On your ViewModel, they can just be IEnumerable<T>, unless you need to be doing a .Add() on them later. That can be whatever you want to be. Also, they do not have to be virtual on the Model because you're not having EF create a derived type of your model (again, unless you have other reason to).

To set them back to what they are in the DTO object, if you just want to set it directly and not check individual properties, you can use .ToList() to have it fulfill the ICollection<T> requirement.

// Assuming "var model" is coming in as a parameter
var station = context.Viewers.First();
station.Broadcasters = model.Broadcasters.Select(b => new User {
    Id = b.Id,
    SkypeId = b.SkypeId,
    Name = b.Name
}).ToList();
station.Viewers = model.Viewers.Select(v => new User {
    Id = v.Id,
    SkypeId = v.SkypeId,
    Name = v.Name
});