One to zero-or-one with HasForeignKey

OK, I figured that out – you should use WithMany (yep, not obvious) in order to store foreign key in some property:

Property(e => e.PersonId).HasColumnName("person_id");
HasRequired(e => e.Person)
    .WithMany()
    .HasForeignKey(p => p.PersonId);

See One-to-One Foreign Key Associations article for details. BTW this will create employee foreign key column for person’s table, but there is no other way to have navigation property and foreign key separately.

If you need foreign key read-only, you can change PersonId property to:

public int PersonId { get { return Person.Id; } }

And use your original mapping

HasRequired(e => e.Person)
    .WithOptional(p => p.Employee)
    .Map(m => m.MapKey("person_id"));