C# 基本表设计中的外键约束异常

我使用的是EntityFrameworkCore，有一个简单的5表设计，但出现了一个错误，无法找出原因。我错过了什么

例外情况：

public abstract class Base
{
    public int Id { get; set; }
    public DateTime DateCreated { get; set; } = DateTime.Now;
}

public class Client : Base
{
    public Guid Key { get; set; }
    public string Name { get; set; }
}

public class Graphic : Base
{
    public int ClientId { get; set; }
    public virtual Client Client { get; set; }
    public ICollection<GraphicItem> Items { get; set; } = new HashSet<GraphicItem>();
    public ICollection<Tag> Tags { get; set; } = new HashSet<Tag>();
}

public class GraphicUploadTemplate : Base
{
    public int ClientId { get; set; }
    public virtual Client client { get; set; }    
    public ICollection<GraphicUploadTemplateItem> Items { get; set; }
}

public class GraphicUploadTemplateItem : Base
{
    public int GraphicUploadTemplateId { get; set; }
    public virtual GraphicUploadTemplate UploadTemplate { get; set; }
}

引入外键约束 'FK_GraphicItems_GraphicUploadTemplateItems_GraphicUploadTemplateItemId' 表上的“GraphicItems”可能导致循环或多个级联路径。 指定在删除时不执行操作或在更新时不执行操作，或修改其他 外键约束

基础：

public abstract class Base
{
    public int Id { get; set; }
    public DateTime DateCreated { get; set; } = DateTime.Now;
}

public class Client : Base
{
    public Guid Key { get; set; }
    public string Name { get; set; }
}

public class Graphic : Base
{
    public int ClientId { get; set; }
    public virtual Client Client { get; set; }
    public ICollection<GraphicItem> Items { get; set; } = new HashSet<GraphicItem>();
    public ICollection<Tag> Tags { get; set; } = new HashSet<Tag>();
}

public class GraphicUploadTemplate : Base
{
    public int ClientId { get; set; }
    public virtual Client client { get; set; }    
    public ICollection<GraphicUploadTemplateItem> Items { get; set; }
}

public class GraphicUploadTemplateItem : Base
{
    public int GraphicUploadTemplateId { get; set; }
    public virtual GraphicUploadTemplate UploadTemplate { get; set; }
}

客户端：

public abstract class Base
{
    public int Id { get; set; }
    public DateTime DateCreated { get; set; } = DateTime.Now;
}

public class Client : Base
{
    public Guid Key { get; set; }
    public string Name { get; set; }
}

public class Graphic : Base
{
    public int ClientId { get; set; }
    public virtual Client Client { get; set; }
    public ICollection<GraphicItem> Items { get; set; } = new HashSet<GraphicItem>();
    public ICollection<Tag> Tags { get; set; } = new HashSet<Tag>();
}

public class GraphicUploadTemplate : Base
{
    public int ClientId { get; set; }
    public virtual Client client { get; set; }    
    public ICollection<GraphicUploadTemplateItem> Items { get; set; }
}

public class GraphicUploadTemplateItem : Base
{
    public int GraphicUploadTemplateId { get; set; }
    public virtual GraphicUploadTemplate UploadTemplate { get; set; }
}

图形：

public abstract class Base
{
    public int Id { get; set; }
    public DateTime DateCreated { get; set; } = DateTime.Now;
}

public class Client : Base
{
    public Guid Key { get; set; }
    public string Name { get; set; }
}

public class Graphic : Base
{
    public int ClientId { get; set; }
    public virtual Client Client { get; set; }
    public ICollection<GraphicItem> Items { get; set; } = new HashSet<GraphicItem>();
    public ICollection<Tag> Tags { get; set; } = new HashSet<Tag>();
}

public class GraphicUploadTemplate : Base
{
    public int ClientId { get; set; }
    public virtual Client client { get; set; }    
    public ICollection<GraphicUploadTemplateItem> Items { get; set; }
}

public class GraphicUploadTemplateItem : Base
{
    public int GraphicUploadTemplateId { get; set; }
    public virtual GraphicUploadTemplate UploadTemplate { get; set; }
}

图形上传模板：

public abstract class Base
{
    public int Id { get; set; }
    public DateTime DateCreated { get; set; } = DateTime.Now;
}

public class Client : Base
{
    public Guid Key { get; set; }
    public string Name { get; set; }
}

public class Graphic : Base
{
    public int ClientId { get; set; }
    public virtual Client Client { get; set; }
    public ICollection<GraphicItem> Items { get; set; } = new HashSet<GraphicItem>();
    public ICollection<Tag> Tags { get; set; } = new HashSet<Tag>();
}

public class GraphicUploadTemplate : Base
{
    public int ClientId { get; set; }
    public virtual Client client { get; set; }    
    public ICollection<GraphicUploadTemplateItem> Items { get; set; }
}

public class GraphicUploadTemplateItem : Base
{
    public int GraphicUploadTemplateId { get; set; }
    public virtual GraphicUploadTemplate UploadTemplate { get; set; }
}

---

通过GraphicItem.UploadTemplateItem.Id将有一个指向相应GraphicUploadTemplateItem的链接，我认为这会导致循环引用。

通过将属性类型设置为nullable，修复了该问题

public class GraphicItem : Base
{
    public int GraphicId { get; set; }
    public virtual Graphic Graphic { get; set; }
    public int? GraphicUploadTemplateItemId { get; set; }
    public virtual GraphicUploadTemplateItem UploadTemplate { get; set; }
}

---

啊，是的，不正常！对不起，我瞎了眼；）谢谢！可能重复的