one to one - EF Code First - 1-to-1 Optional Relationship

Question

I want to map an optional 1-to-1 relationship in an existing database with EF Code First.

Simple schema:

User
 Username
 ContactID

Contact
 ID
 Name

Obviously ContactID joins to Contact.ID. The ContactID field is nullable so the relationship is optional - 0 or 1, never many.

So how do I specify this relationship in EF Code First, with this existing schema?

Here's what I've tried so far:

public class User
{
    [Key]
    public string Username { get; set; }
    public int? ContactID { get; set; }

    [ForeignKey("ContactID")]
    public virtual Contact Contact { get; set; }
}

public class Contact
{
    [Key]
    public int ID { get; set; }
    public string Name { get; set; }

    public virtual User User { get; set; }
}

modelBuilder.Entity<User>().HasOptional<Contact>(u=> u.Contact)
    .WithOptionalDependent(c => c.User);

I get the following Exception:

  System.Data.Edm.EdmAssociationEnd: : Multiplicity is not valid in Role
 'User_Contact_Source' in relationship 'User_Contact'. Because the Dependent 
Role properties are not the key properties, the upper bound of the multiplicity 
of the Dependent Role must be *.

Answer 1

One solution would be;

public class User
{
    [Key]
    public string Username { get; set; }

    public virtual Contact Contact { get; set; }
}

public class Contact
{
    [Key]
    public int ID { get; set; }
    public string Name { get; set; }

    public virtual User User { get; set; }
}

        modelBuilder.Entity<User>()
            .HasOptional<Contact>(u => u.Contact)
            .WithOptionalDependent(c => c.User).Map(p => p.MapKey("ContactID"));

You set only your navigational objects in your POCOs and instead you use fluent API to map your key to the correct column.