I have two tables Employer and OldEmployer. The employer table has a column old_id that is a foreign key to OldEmployer.Id. But not every Employer has a corresponding record in OldEmployer so old_id is sometimes null.



I need a one to one mapping between these two tables where the foreign key is sometimes null. How do i do this?

Use the "OldEmployee" property at the "Employee" side and map it as <many-to-one name="OldEmployee" column="old_id" class="OldEmployer"> (not one-to-one!).
On the old one side use <one-to-one name="CurrentEmployee" class="Emplayee" property-ref="OldEmployer"> (no extra column at the old one side - PK is used)

thx