Entity Framework 5.0b2 Code First: One-To-Many and

问题:

After several hours of trial and error, I reached to this thread which explains how to establish a One-To-Many relationship and a One-To-One relationship with the same two types.

However, I cannot get this to work with Cascade Delete:

Thrown: "Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values." (System.Data.UpdateException) Exception Message = "Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.", Exception Type = "System.Data.UpdateException"

This only happens if I don't unset the 1:1 relationship (see code below), which I guess makes sense given that it would create an invalid reference. I'm just wondering if there is a better way to represent this.

Sample code:

class Program
{
    static void Main(string[] args)
    {
        Database.SetInitializer(new DropCreateDatabaseAlways<Context>());

        using (var ctx = new Context())
        {
            var user = new User();

            ctx.Users.Add(user);
            ctx.SaveChanges();

            var source = new PaymentSource();
            user.PaymentSources = new Collection<PaymentSource>();
            user.PaymentSources.Add(source);
            user.DefaultPaymentSource = source;
            ctx.SaveChanges();

            // if I don't do this, I get ordering exception
            user.DefaultPaymentSource = null;
            ctx.SaveChanges();

            ctx.Users.Remove(user);
            ctx.SaveChanges();

            Assert.Equal(0, ctx.Users.Count());
            Assert.Equal(0, ctx.PaymentSources.Count());
        }
    }
}

public class User
{
    public int Id { get; set; }

    public virtual ICollection<PaymentSource> PaymentSources { get; set; }
    public virtual PaymentSource DefaultPaymentSource { get; set; }
    public int? DefaultPaymentSourceId { get; set; }
}

public class PaymentSource
{
    public int Id { get; set; }
    public virtual User User { get; set; }
    public int UserId { get; set; }
}

public class Context : DbContext
{
    public DbSet<User> Users { get; set; }
    public DbSet<PaymentSource> PaymentSources { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<User>()
            .HasOptional(u => u.DefaultPaymentSource)
            .WithMany()
            .HasForeignKey(u => u.DefaultPaymentSourceId)
            .WillCascadeOnDelete(false);

        modelBuilder.Entity<PaymentSource>()
            .HasRequired(p => p.User)
            .WithMany(p => p.PaymentSources)
            .HasForeignKey(p => p.UserId)
            .WillCascadeOnDelete();
    }
}

回答1:

I listed other options to describe your abstraction:

A.

How about using 3 tables like that:

user 1-* paymentSource
user 1-0..1 DefaultPaymentSource
DefaultPaymentSource 0..1-1 PaymentSource

or this:

B.

user 1-* paymentSource
user 1-0..1 DefaultPaymentSource
DefaultPaymentSource --derive from--> PaymentSource

or this:

C.

user 1-* paymentSource
PaymentSource has addtional boolean field for "IsDefault"

I vote for choice B as best one.

I am certain that having two relations comming from the same source table to the same destination table is not going to be a good idea.. it is probably breaking some rule or pattern regarding databases best practice.