.NET Entity Framework Core 2.0 - 代码优先 - 意外的表关系结果

Question

I'm learning EF Core and put wrong code in the Fluent API, which resolved to very strange One-to-One Relationship in the created database. Let me give you some code and more specific information and I really hope someone can explain how this happen.

I have coded 3 Models in C# and 1 Mapping table. The problem occurred between 2 of the models.

public class Album
{
    [Key]
    public int AlbumId { get; set; }

    public string BackgroundColor { get; set; }

    public Boolean IsPublic { get; set; }

    public int PhotographerId { get; set; }

    public Photographer Photographer { get; set; }

    public IList<PictureAlbum> AlbumPictures { get; set; } = new List<PictureAlbum>();
}

public class Photographer
{
    [Key]
    public int PhotographerId { get; set; }

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

    [Required]
    [MinLength(6)]
    public string Password { get; set; }

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

    [Required]
    public DateTime RegisteredDate { get; set; }

    [Required]
    public DateTime BirthDate { get; set; }

    public IList<Album> Albums { get; set; } = new List<Album>();
}

So everything looks right. 'Album' have Photographer and Foreign Key. Photographer have Collection of 'Albums'.

Here is part of the table relationship in the Fluent API (included only the relationship for the "strange" relationship between Album and Photographer):

builder.Entity<Album>()
            .HasOne(p => p.Photographer)
            .WithMany(a => a.Albums)
            .HasForeignKey(p => p.AlbumId);

As you can see, instead of putting "PhotographerId" for the ForeignKey, I put "AlbumId" which should leads to "Self-Referencing" Table, right?

But this looks not true, because when I review the Diagram of the Database I see the following:

Diagram (Relation between Album and Photographer)

More of that, if you look in the Key's they have, they looks like they are coded in C#:

Tables Keys

Now I do not understand how this is possible. I created the Database using Migrations.

1. I did not put the right Foreign Key in FluentApi, but the Diagram shows One-to-One Relationship?
2. In the Keys of the table, we can see that they don't have Relationship.
3. More than that I have IList Albums in "Photographer" Model, which by my understanding should lead to may be something different, but not One-to-One.

I know I made a mistake with the FluentApi, but I want to learn from my mistake and to understand how this result happened.

This is my first post here and I hope I can get some support/help.

Thank you.

Answer 1

It's a weird situation. You effectively configured a one-to-one association between Photographer (principal) and Album (dependent). How this happened becomes clear by using the correct range variables ( p and a ) in the mapping:

builder.Entity<Album>()
        .HasOne(a => a.Photographer)
        .WithMany(p => p.Albums)
        .HasForeignKey(a => a.AlbumId); // i.e. this is Album.AlbumId

Album.AlbumId now is Album s primary key and its foreign key to Photographer . The primary key is not an identity field, because it "borrows" its value from the owning Photographer . This is a common way of modeling 1:1 associations in a relational database. But for EF, the association is 1-n and it's a bit surprising that it doesn't give a warning about this anomaly.

The funny thing is that is still works. You can establish the 1:1 association between a Photographer and an Album by adding the album to Photographer.Albums , and EF saves everything alright. However, if you try to add a second Album , EF will run into a duplicate primary key exception.