Navigation Properties using two foreign keys - entity-framework

Question

I have a project with several tables in the same database.
public class UserImage
{
[Key]
public int Id { get; set; }
public string OwnerId { get; set; }
[ForeignKey("OwnerId")]
public virtual ApplicationUser Owner { get; set; }
//some fields are removed for brevity
}
public class FriendRequest
{
[Key]
public int Id { get; set; }
public string UserId { get; set; }
public string FutureFriendUserId { get; set; }
[ForeignKey("UserId")]
public virtual ApplicationUser User { get; set; }
[ForeignKey("FutureFriendUserId")]
public virtual ApplicationUser FutureFriendUser { get; set; }
}
public class ApplicationUser : IdentityUser
{
//some fields are removed for brevity
public virtual ICollection<UserImage> UserImages { get; set; }
public virtual ICollection<FriendRequest> FriendRequests { get; set; }
The problem is that I can find the images that belong to a user:
var userStore = new UserStore<ApplicationUser>(db);
var userManager = new UserManager<ApplicationUser>(userStore);
ApplicationUser user = userManager.FindById(User.Identity.GetUserId());
IEnumerable<string> imgs = (from image in user.UserImages select Url.Content(image.ImageUrl)).Skip(skip).Take(5).ToList();
but I can't use the same technique for the FriendRequests. If I search in the database for the rows that have UserId == User.Identity.GetUserId() or some other id, the results are what I expect.
What is the problem?

Answer 1

What you're essentially creating here is a self-referential many-to-many relationship. On your FriendRequest class, you have two properties that are foreign keys to ApplicationUser, but on your ApplicationUser class, you have only a single collection of FriendRequest. Entity Framework has no idea which foreign key should actually compose this collection. As a result, you have to make a few changes to get this working properly.
You must add another navigation property. Essentially, on your ApplicationUser class you'll end up with something like the following:
public virtual ICollection<FriendRequest> SentFriendRequests { get; set; }
public virtual ICollection<FriendRequest> ReceivedFriendRequests { get; set; }
Again, you need a collection for each foreign key.
You'll need to add some fluent config to help Entity Framework determine which foreign key to use for each collection:
public class ApplicationContext : DbContext
{
...
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<FriendRequest>().HasRequired(m => m.User).WithMany(m => m.SentFriendRequests);
modelBuilder.Entity<FriendRequest>().HasRequired(m => m.FutureFriendUser).WithMany(m => m.ReceivedFriendRequests);
}