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

问题

I know there are some answered questions on SO about 1:0..1-relationships. I have looked at this and this, but don't think they apply to my question.

I have these three (simplified) models in a CMS-system.

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; }
}

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.

In an attempt to configure the relationships, I have added this bit of code:

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);
}

But I don't think it's correct. I haven't made the CRUD-views for FrontPageItem yet, but if I try to add items directly in the SQL Server Object Explorer in VS, I have to enter a value for the PK.

What am I doing wrong?

回答1:

As you said:

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.

Then remove ArticleId and WebPageId from FrontPageItem as EF core support one-to-one-or-zero association without foreign key in principle 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; }
}

Then the Fleunt API configuration for Article and WebPage as follows:

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
}

来源：https://stackoverflow.com/questions/55889449/how-to-configure-this-one-to-zero-or-one-relationship