Mapping foreign key in HasOptional().WithOptionalDependent() relation in Entity Framework 6

Question

I have the following data-model in Entity Framework 6.1.3:

using System.Data.Entity;

public class Student
{
    public int Id { get; set; }
    public virtual Contact Contact { get; set; }
}

public class Contact
{
    public int Id { get; set; }
    public virtual Student Student { get; set; }
}

public class MyContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder builder)
    {
        builder.Entity<Contact>()
            .HasOptional(x => x.Student)
            .WithOptionalDependent(x => x.Contact)
            .WillCascadeOnDelete(true);
    }
}

public static class Program
{
    private static void Main()
    {
        Database.SetInitializer(new DropCreateDatabaseAlways<MyContext>());

        using (var context = new MyContext())
            context.Database.Initialize(force: true);
    }
}

When I launch this code, I get exactly the right table structure I am aiming for:

dbo.Contacts
    Id (PK)
    Student_Id (FK, NULL, CASCADE ON DELETE)

dbo.Students
    Id (PK)

However, now I would like to add the Student_Id property to be available in the Contact entity. So I can read the Student_Id without needing to join the other table through .Student.Id navigation.

If I add the property to the Contact entity, I end up either with two columns Student_Id and Student_Id1, or I end up with an error message saying Each property name in a type must be unique..

The column is already in the database, all I need is to have it in the entity as well, why is it so much trouble? Is there a solution?

Answer 1

I managed to get a response from the Entity Framework Program Manager after asking on GitHub.

Unfortunately this is a limitation of EF6. You can not have a foreign key property in a one-to-one relationship, unless it is also the primary key property. This is essentially because EF6 doesn't support alternate keys/unique indexes, so you can't enforce that a non-primary key property is unique. The fact that you can do it when the foreign key property isn't in the entity is a bit of a quirk... but obviously not something we would remove 😄.

BTW alternate keys (and therefore this scenario) is supported in EF Core.

– Rowan Miller @ https://github.com/aspnet/EntityFramework6/issues/159#issuecomment-274889438