gistfile1.txt

using System;
using System.Linq;
using Microsoft.Data.Entity;

namespace Test
{
    public static class Program
    {
        static void Main(string[] args)
        {
            using (var db = new Context())
            {
                var a = new A
                {
                    Field1 = 13
                };

                db.Add(a);

                var b = new B
                {
                    Field1 = 14
                };

                a.B = b;
                db.SaveChanges();
                var result = db.As.First();
                Console.WriteLine(result.BId ?? -1);
            }
        }

        public class Context : DbContext
        {
            public DbSet<A> As { get; set; }
            public DbSet<B> Bs { get; set; }

            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
            {
                optionsBuilder.UseInMemoryDatabase();
            }

            protected override void OnModelCreating(ModelBuilder modelBuilder)
            {
                modelBuilder.Entity<B>(entity =>
                {
                    entity.HasOne(d => d.A)
                        .WithOne(p => p.B)
                        .HasForeignKey<A>(k => k.BId);
                });
            }
        }

        public class A
        {
            public int Id { get; set; }
            public int Field1 { get; set; }
            public int? BId { get; set; }
            public B B { get; set; }
        }

        public class B
        {
            public int Id { get; set; }
            public int Field1 { get; set; }
            public A A { get; set; }
        }
    }
}