<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" default-access="field">
  <class name="SpringAir.Parts.PartOut, SpringAir.Objects" table="PartOut">
    <id name="partOutId" type="Int32" column="PartOutId" unsaved-value="-1">
       <generator class="hilo"><!--... blah, blah ... --></generator>
    </id>
    <many-to-one name="partIn" column="PartInId" cascade="all" unique="true" />
  </class>

  <class name="SpringAir.Parts.PartIn, SpringAir.Objects" table="PartIn">
    <id name="partInId" type="Int32" column="PartInId" unsaved-value="-1">
       <generator class="hilo"><!--... blah, blah ... --></generator>
    </id>
    <one-to-one name="partOut" property-ref="partIn" cascade="none" />
  </class>
</hibernate-mapping>

Table PartIn:
-------------------
| PartInId (int)  |
-------------------
Table PartOut:
--------------------------------------
| PartOutId (int)  |  PartInId (int) |
--------------------------------------

public class PartOut {
  int partOutId;
  PartIn partIn;
}
public class PartIn {
  int partInId;
  PartOut partOut;
}