Strange behavior with HasRequired

Question

The help for HasRequired states

Configures a required relationship from this entity type. Instances of the entity type will not be able to be saved to the database unless this relationship is specified. The foreign key in the database will be non-nullable.

I have a none required field configured like

public class BookingConfiguration : EntityTypeConfiguration<Booking>
    {
        public BookingConfiguration()
        {
            HasKey(ai => ai.BookingKey);

            ...

            HasRequired(b => b.Customer)
                .WithRequiredPrincipal();

            ToTable("Booking");
        }
    }

This is ofcourse very strange since Customer is not required. But, it works :D Both for null values and when customer is set. If I change to

HasOptional(b => b.Customer)
    .WithOptionalPrincipal();

It fails with (Already at get from database)

Invalid column name 'Booking_BookingKey'.

The code that fails

var existingBooking = await _context.DbSet<Booking>().FirstAsync(ai => ai.BookingKey == confirmCommand.BookingKey);

edit: Structure of entities are

public class Booking
{
   public Guid BookingKey { get; set; }
   ...
   public Customer Customer { get; set; }
}

public class Customer
{
   public Guid BookingKey { get; set; }
   ...   
}

Answer 1

Since BookingKey is both the Key for Booking and the ForeignKey for Customer try setting the ForeignKey field during setup.

HasOptional(c => c.Customer)
     .WithOptionalPrincipal()
     .Map(m => m.MapKey("BookingKey"));