public class A {
  int aId;
  IList log;
  //...
  public class B {
    int bId;
    //...
  }
}
---
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0" default-access="field">
   <class name="A, SpringAir.Objects" table="A">
      <!-- ... standard A defs ... -->
      <bag name="log" lazy="true" cascade="all" table="B">
         <key column="BId" />
         <composite-element class="A.B, SpringAir.Objects">
            <!-- B defs -->
         </composite-element>
      </bag>
   </class>
</hibernate-mapping>

System.TypeLoadException: Could not load type A.B from assembly SpringAir.Objects

<class name="SpringAir.Objects.A, SpringAir.Objects" table="A">