EF Code First 4.1 - How to configure one to many relationship with default

Question

I have a Customer entity which references a collection of Addresses. The complication here is that I want to be able to identify a particular address as the default address

Answer 1

I don't understand what EF is talking there about "not exposed foreign keys" in the exception. I would consider the inner exception as the important part:

Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.

I think the problem in your model is that you have a mutual dependency between Customer and Address: An address needs a customer (you have marked it as required in your mapping code) and on the other hand a customer needs an address (the default address is required both due to the non-nullable foreign key and due to your mapping code). So, EF doesn't know which entity to save first in your example code - the default address or the customer? Both entities need the primary key of the other to be saved with valid FK contraints.

The easiest solution I can see is to make the default address optional in your model and then save twice (I omit the mappings which work by convention anyway):

public class Customer
{
    private ICollection<Address> m_Addresses;

    public Customer() { Addresses = new List<Address>(); }

    public int Id { get; set; }
    public string CompanyName { get; set; }
    public virtual ICollection<Address> Addresses { get { ... } set { ... } }
    public Address DefaultAddress { get; set; }
    public int? DefaultAddressId { get; set; } // FK for optional relationship
}

public class Address
{
    public int Id { get; set; }
    public string Town { get; set; }
    public Customer Customer { get; set; }
}

// ...

public class CustomerConfiguration : EntityTypeConfiguration<Customer>
{
    public CustomerConfiguration() : base()
    {
        Property(p => p.CompanyName)
            .HasColumnName("Name")
            .IsRequired();

        HasMany(c => c.Addresses)
            .WithRequired(a => a.Customer)
            .Map(x => x.MapKey("CustomerId"));
    }
}

public class AddressConfiguration : EntityTypeConfiguration<Address>
{
    public AddressConfiguration() : base()
    {
        Property(p => p.Town)
            .HasColumnName("Town")
            .IsRequired();
    }
}

And then your program would look like this:

static void Main(string[] args)
{
    Customer headOffice = new Customer();
    headOffice.CompanyName = "C1";

    Address address = new Address();
    address.Town = "Colchester";
    headOffice.Addresses.Add(address);

    address = new Address();
    address.Town = "Norwich";
    headOffice.Addresses.Add(address);

    //headOffice.DefaultAddress = address;
    //We don't set the default address here as SaveChanges would throw an
    //exception. But because it is optional now we are allowed to leave it null.

    MyContext context = new MyContext(ConnectionString);
    context.Customers.Add(headOffice);
    context.SaveChanges();

    headOffice.DefaultAddress = address; // headoffice and address have now PKs
    context.SaveChanges(); // Updates headoffice in the DB with default address
}

This double SaveChanges is ugly, but I don't see another way.