Sequence contains no elements : self referencing code first

问题

when I try to do self referencing with next entities

public class Folder
{
    public int Id { get; set; }
    public int?  ParentFolderId { get; set; } // Added property
    public string Name { get; set; }


    public virtual ICollection<Folder> Childrens { get; set; } // Added property
    public virtual ICollection<ArticleTitle> ArticleTitles { get; set; }

}


public class ArticleTitle
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int? FolderId { get; set; }
    public virtual Folder Folder { get; set; }
    public virtual Article Article { get; set; }
}

I added few properties (commented 'added property') and this line of code:

  modelBuilder.Entity<Folder>()
 .HasMany(s => s.Childrens)
 .WithOptional()
 .HasForeignKey(s => s.ParentFolderId);

And get this exception:

Sequence contains no elements

Is it not correct self referencing?

回答1:

This is how your code should look like:

public class Folder
{
    public int Id { get; set; }
    [ForeignKey("ParentFolder")]
    public int?  ParentFolderId { get; set; }
    public virtual Folder ParentFolder { get; set; }
    public string Name { get; set; }


    public virtual ICollection<Folder> Childrens { get; set; }
    public virtual ICollection<ArticleTitle> ArticleTitles { get; set; }

}
...

I'm glad it helped.

来源：https://stackoverflow.com/questions/15857203/sequence-contains-no-elements-self-referencing-code-first