Question

I’ve created a relationship on the same table, and the relationship is declared in the OnModelCreating. See code below.

When trying to get all items with the following code var allitems = MyContext.WallItems
I get the following error: Server Error in ‘/’ Application.
Unable to cast object of type ‘WallItem’ to type ‘System.Collections.Generic.List`1[WallItem]’.

Have i defined my relationship in a wrong way, or is there a better way?

[Table("WallItems")]
public class WallItem
{
    public WallItem() { Id = Guid.NewGuid(); }

    /// <summary>
    /// Item Id
    /// </summary>
    [Key]
    public Guid Id { get; set; }

    /// <summary>
    /// Enables replies on wall items. Links to the root item.
    /// </summary>
    public Guid? ThreadRoot { get; set; }

    /// <summary>
    /// Collection of replies
    /// </summary>
    public virtual List<WallItem> Comments { get; set; }

}

public class MyContext : DbContext
{
    public DbSet<WallItem> WallItems { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<WallItem>()
                            .HasOptional(c => c.Comments)
                            .WithMany()
                            .HasForeignKey(c => c.ThreadRoot);
    }
}

Answer 1

You have mistake in your Fluent API configuration. This will work:

modelBuilder.Entity<WallItem>()
    .HasMany(c => c.Comments)
    .WithOptional()
    .HasForeignKey(c => c.ThreadRoot);