docs: private ReferenceDescriptorDef usedByReference(ModelDef modelDef, FieldDescriptorDef fieldDef)
      {
          String                 ownerClassName = ((ClassDescriptorDef)fieldDef.getOwner()).getQualifiedName();
          ClassDescriptorDef     classDef;
          ReferenceDescriptorDef refDef;
          String                 targetClassName;
  
          // only relevant for primarykey fields
          if (PropertyHelper.toBoolean(fieldDef.getProperty(PropertyHelper.OJB_PROPERTY_PRIMARYKEY), false))
          {
              for (Iterator classIt = modelDef.getClasses(); classIt.hasNext();)
              {
                  classDef = (ClassDescriptorDef)classIt.next();
                  for (Iterator refIt = classDef.getReferences(); refIt.hasNext();)
                  {
                      refDef          = (ReferenceDescriptorDef)refIt.next();
                      targetClassName = refDef.getProperty(PropertyHelper.OJB_PROPERTY_CLASS_REF).replace('$', '.');
                      if (ownerClassName.equals(targetClassName))
                      {
                          // the field is a primary key of the class referenced by this reference descriptor
                          return refDef;
                      }
                  }
              }
          }
          return null;
      }