Entity Framework: ono to one relation

Example:

public class User
{
    public int UserId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }      
    public int BillingAddressId { get; set; }

    public Address BillingAddress { get; set; }

}

public class Address
{
    public int AddressId { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
    public string PostalCode { get; set; }
}

// one user one billing adress
modelBuilder.Entity<User>()
                .HasRequired(a => a.BillingAddress)
                .WithMany()
                .HasForeignKey(u => u.BillingAddressId);

Note: this is not going to generate the unique constraint in the db on  BillingAddressId field

Reference:
http://weblogs.asp.net/manavi/archive/2011/01/23/associations-in-ef-code-first-ctp5-part-3-one-to-one-foreign-key-associations.aspx