如何配置这种一对零或一的关系？

Question

我知道在 SO 上有一些关于 1:0..1 关系的已回答问题。我看过 this and ，但认为它们不适用于我的问题。

我在 CMS 系统中有这三个（简化的）模型。

public class FrontPageItem
{
    public int Id { get; set; }
    public int ItemType { get; set; } // 1 = Article, 2 = WebPage, etc...

    public int? ArticleId { get; set; }
    public Article Article { get; set; }

    public int? WebPageId { get; set; }
    public WebPage WebPage { get; set; }
}

public class Article
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Preamble { get; set; }
    public string MainText { get; set; }

    public int? FrontPageItemId { get; set; }
    public FrontPageItem FrontPageItem { get; set; }
}

public class WebPage
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string Content { get; set; }

    public int? FrontPageItemId { get; set; }
    public FrontPageItem FrontPageItem { get; set; }
}

FrontPageItem 与每个不同元素类型之间的关系是一对零或一对一的关系。一个元素可以在不添加为 FrontPageItem 的情况下存在，这意味着 FrontPageItem 只与一个元素有关系，可以是 Article 也可以是 WebPage.

为了尝试配置关系，我添加了这段代码：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Article>()
        .HasOne(p => p.FrontPageItem)
        .WithOne(i => i.Article)
        .HasForeignKey<FrontPageItem>(b => b.Id);
    modelBuilder.Entity<WebPage>()
        .HasOne(p => p.FrontPageItem)
        .WithOne(i => i.WebPage)
        .HasForeignKey<FrontPageItem>(b => b.Id);
}

但我认为这是不正确的。我还没有为 FrontPageItem 创建 CRUD 视图，但是如果我尝试直接在 VS 的 SQL 服务器对象资源管理器中添加项目，我必须为 PK 输入一个值。

我做错了什么？

Answer 1

正如你所说：

The relationships between a FrontPageItem and each of the different element types are one-to-zero-or-one. An element can exist without being added as a FrontPageItem, meaning that a FrontPageItem has a relationship to just one element, either an Article or a WebPage.

然后从FrontPageItem中删除ArticleId和WebPageId作为EF核心支持one-to-one-or-zero原则上没有外键的关联table:

public class FrontPageItem
{
    public int Id { get; set; }
    public int ItemType { get; set; } // 1 = Article, 2 = WebPage, etc...

    public Article Article { get; set; }
    public WebPage WebPage { get; set; }
}

然后Fleunt API配置Article和WebPage如下：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Article>()
        .HasOne(a => a.FrontPageItem)
        .WithOne(f => f.Article)
        .HasForeignKey<Article>(a => a.FrontPageItemId); // <-- Here it is

    modelBuilder.Entity<WebPage>()
        .HasOne(wp => wp.FrontPageItem)
        .WithOne(f => f.WebPage)
        .HasForeignKey<WebPage>(wp => wp.FrontPageItemId); // <--Here it is
}

如何配置这种一对零或一的关系？

How to configure this one-to-zero-or-one relationship?

c#

ef-code-first

entity-framework-core