Entity framework code first delete entities without parent - entity-framework

Question

I have two entities Users and Phone. Relationship is one to one. Mapping is
public class User
{
public virtual Phone Phone { get; set;}
public virtual int PhoneId { get; set;}
}
public class Phone
{
public virtual string Number { get; set;}
}
My mapping for User is:
HasRequired(x => x.Phone).WithMany().HasForeignKey(x => x.PhoneId)
I have user with assigned phone.
f.e.
Phone oldPhone = new Phone();
Phone newPhone = new Phone();
User user = new User();
user.Phone = old;
///Save user with phone.
user.Phone = newPhone;
///Save user with phone.
Now I have user with assigned phone - newPhone and not assigned oldPhone row in the DB.
How can I setup mapping to delete entity without parent(oldPhone).
EDITED.
I have changed mapping according this article to
HasRequired(x => x.Phone).WithOptional();
For save I use this method:
public override void Save(TEntity entity)
{
if (entity == null)
{
throw new ArgumentNullException("entity");
}
if (entity.Id > 0)
{
DbContext.Set<TEntity>().Attach(entity);
DbContext.Entry(entity).State = System.Data.EntityState.Modified;
}
else
{
DbContext.Set<TEntity>().Add(entity);
}
}
When I attached new entity old ones doesn`t deleted from DB, so in my example I have two entities in DB oldPhone and newPhone.

Answer 1

I don't know a way to do this through mapping in EF. For one of my projects I have made a Parent attribute, which I could check on Save. I think you could use it here to. I have made a blog post on this with code; extending entity framework 4 with parentvalidator.
It would require to add the User property to the Phone entity.
[Parent]
public virtual User User { get; set;}