NHibernate mapping returns null value - nhibernate

Question

I have a problem with mapping in NHibernate.
The Order table has the Invoice_Id column which is the nullable FK to the Invoice table.
The problem is, when I load an Invoice which Id exists in the Order table, I see that ConnectedOrder property is null, why?
public class Invoice
{
public virtual Order ConnectedOrder { get; set; }
}
public class Order
{
public virtual Invoice ConnectedInvoice { get; set; }
}
public class InvoiceMap : ClassMap<Invoice>
{
public InvoiceMap()
{
this.References(x => x.ConnectedOrder).Nullable();
}
}
public class OrderMap : ClassMap<Order>
{
public OrderMap()
{
this.References(x => x.ConnectedInvoice).Nullable();
}
}
edit
I've changed my classes and mappings like Radim Köhler said, then I found that topic
Fluent NHibernate One-To-Many Mapping
and there was the need to also add:
this.HasMany(x => x.Orders)
.KeyColumn("Invoice_id")
.Inverse()
.Cascade
.AllDeleteOrphan();
and now it works

Answer 1

You may not like it, but the table structure described above, is not representing Entity relations you've created (so called one-to-one).
In case, that one table contains column referencing the another table (FK), we have scenario:
Each Order has exactly one (or null) Invoice. (many-to-one)
Invoice can be referenced by none or one or many Orders. (one-to-many)
That means, that we should express Entities like this:
public class Invoice
{ // many orders could reference us
public virtual IList<Order> Orders { get; set; }
...
public class Order
{ // unchanged
public virtual Invoice ConnectedInvoice { get; set; }
...
And the mapping should be:
public InvoiceMap()
{ // HasMany is one-to-many
this.HasMany(x => x.Orders)
...
}
public OrderMap()
{ // References is many-to-one
this.References(x => x.ConnectedInvoice).Nullable();
...