如何在 Entity Framework Core 中定义外键

Question

我正在尝试实现以下 table 结构，但出现错误 “不能用作实体类型的 属性，因为它被配置为导航。”

谁能帮帮我。

类别：

Id	Name
1	cat1
2	cat2
3	cat3
4	cat4

子类别：

Id	Category Id	Name
1	1	Subcat1
2	1	Subcat2
3	1	Subcat3
1	2	Subcat4
1	3	Subcat5
2	3	Subcat6

项目：

Id	CategoryId	SubCategoryId	Name
1	1	2	item1
2	3	2	item2
3	1	null	item3
4	4	null	item4

public class Category
{
    [Key]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    public virtual ICollection<HrCodeSubType> SubCategories { get; set; }

}

public class SubCategory
{
    [Key]
    public int Id { get; set; }

    [ForeignKey("Category")]
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }

    [Required]
    public string Name { get; set; }
}

public class Item
{
    [Key]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    [ForeignKey("Category")]
    public int CategoryId { get; set; }
    public virtual Category Category{ get; set; }


    public int? SubCategoryId { get; set; }
    public virtual SubCategory SubCategory { get; set; }
  }

流利Api

builder.Entity<Item>().HasOne(x => x.SubCategory)
         .WithMany().HasForeignKey(x => new { x.CategoyId, x.SubCategoyId });

Answer 1

只保留项目中的子类别class，因为它已经依赖于类别并且具有一对一关系

public class Item
{
    [Key]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    
    public int? SubCategoryId { get; set; }
    public virtual SubCategory SubCategory { get; set; }
  }

如果您使用 net core 5+，则不需要任何流畅的 api，只需将导航 属性 添加到子类别

public class SubCategory
{
    [Key]
    public int Id { get; set; }

    [ForeignKey("Category")]
    public int CategoryId { get; set; }
    public virtual Category Category { get; set; }

    [Required]
    public string Name { get; set; }

   public virtual ICollection<Item> Items{ get; set; }

}

public class Category
{
    [Key]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    public virtual ICollection<SubCategory> SubCategories { get; set; }

}