序列不包含任何元素：自參照代碼首先

Question

當我嘗試做自我參照與未來實體

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; } 
} 

我加了一些屬性（評論「添加屬性」），這行代碼：

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

並得到此異常：

序列不包含任何元素

這是不正確的自引用？

Answer 1

這是你的代碼應該如何看起來像：

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; } 

} 
... 

我很高興它幫助。