How to define foreign Key in Entity Framework Core-CodePudding

I am trying to achieve the below table structure, but I am getting error "cannot be used as a property on entity type because it is configured as a navigation."

Can anyone help me.

Category:

Id	Name
1	cat1
2	cat2
3	cat3
4	cat4

Sub Category:

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

Item:

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; }
  }

Fluent Api

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

CodePudding user response：

leave only subcategory in the item class since it depends on category already and has one to one relation

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; }
  }

and if you use net core 5 you don't need any fluent apis, just add navigaion property to subcategory

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; }

}