ASP.NET MVC - 无法在对象中插入重复的键行

Question

我对 Entity Framework Core 有疑问。我有 2 个模型： 分类.cs

    public int Id { get; set; }
    [DisplayName("Category | ")]
    [Required(ErrorMessage = "Category Name is Required")]
    public string CategoryName { get; set; }

还有LibraryItem.cs

    public int Id { get; set; }

    [DisplayName("Category Id")]
    [Required(ErrorMessage = "Category Id is Required")]
    public int CategoryId { get; set; }

    public Category Category { get; set; }
    [Required(ErrorMessage = "Title is Required")]
    public string Title { get; set; }

    [DisplayName("Author, Speaker or Director")]
    [Required(ErrorMessage = "Creator of the media is required")]
    public string Author { get; set; }

    public int? Pages { get; set; }

    [DisplayName("DVD or Audiobook Length")]
    public int? RunTimeMinutes { get; set; }

    [DisplayName("Available to borrow")]
    public bool IsBorrowable { get; set; }

    [DisplayName("Name of Borrower")]
    public string Borrower { get; set; }

    [DisplayName("Date of the Borrow")]
    public DateTime? date { get; set; }

    [DisplayName("Type of Media")]
    [Required(ErrorMessage = "Type of Media is Required")]
    public string Type { get; set; }

LibraryItem 有一个外键 CategoryId，它是 Category 表的主键。 我想添加两个具有相同外键的项目。所以这两个项目属于同一个类别。 但是当我尝试使用相同的 CategoryId 添加另一个时，我收到了这个错误

cannot insert duplicate key row in object

有谁知道是什么导致了这个问题？ 非常感谢！

Answer 1

问题是你创建了一对一的关系，但你需要一对多

你必须修复你的类别类：

    public int Id { get; set; }
    [DisplayName("Category | ")]
    [Required(ErrorMessage = "Category Name is Required")]
    public string CategoryName { get; set; }

    [InverseProperty(nameof(Library.Category))]
    public virtual ICollection<LibraryItem> LibraryItems {get; set:}

修复后您将不得不再次进行迁移

Answer 2

您必须通过在每个类上添加导航属性来让 EF 知道这两个实体以一对多的关系相关：

    public class Category {
        ...
        
        public virtual ICollection<LibraryItem> LibraryItems {get; set;}
    }


    public class LibraryItem {
        ...
        
        [DisplayName("Category Id")]
        [Required(ErrorMessage = "Category Id is Required")]
        public int CategoryId { get; set; }

        public Category Category { get; set; }
    }

如果添加导航属性，则可以使用 Fluent API 在 DbContext 的 OnModelCreating 方法中手动配置关系

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<LibraryItem>()
            .HasOne(l => l.Category)
            .WithMany(c => c.LibraryItems);
    }