Entity Framework code first woes
public class A{
  [Key]
  string Name;
  B DefaultB;
  ICollection<B> Bs;
}

public class B{
  [Key]
  int Key;
}

public class C{
  [Key]
  string Key;
  A MyA;
  B MyB;

  public C(A a,B b){
    MyA=a;
    MyB=b;
  }
}

public class MyDbContext:DbContext{
  public DbSet<A> As
  public DbSet<B> Bs
  public DbSet<C> Cs
}

main(){
  A a=new A();
  B b=new B();
  a.addB(B);
  MyDbContext m=new MyDbContext();
  m.As.Add(a);
  m.SaveChanges();
  C c=new C(a,b);
  m.Cs.Add(c);
  m.SaveChanges();
}

A a=new A();
B b=new B();
a.addB(B);

using (MyDbContext m = new MyDbContext())
{
   m.As.Add(a);

   C c=new C(a,b);
   m.Cs.Add(c);

   m.SaveChanges();
}

A a=new A();
B b=new B();
a.addB(B);

using (MyDbContext m = new MyDbContext())
{
   m.As.Add(a);
   m.SaveChanges();
}

// ...

using (MyDbContext m2 = new MyDbContext())
{
   m2.As.Attach(a);
   m2.Bs.Attach(b);

   C c=new C(a,b);
   m2.Cs.Add(c);

   m2.SaveChanges();
}