code: @Override
  	public PropertyMap getGroupedProperties(SecurityContext securityContext, GraphObject source) {
  
  		if(source instanceof AbstractRelationship) {
  
  			AbstractRelationship rel = (AbstractRelationship)source;
  			PropertyMap properties   = new PropertyMap();
  
  			for (PropertyKey key : propertyKeys.values()) {
  
  				Reference reference = (Reference)key;
  
  				GraphObject referencedEntity = reference.getReferencedEntity(rel);
  				PropertyKey referenceKey     = reference.getReferenceKey();
  				PropertyKey propertyKey      = reference.getPropertyKey();
  
  				if (referencedEntity != null) {
  
  					properties.put(propertyKey, referencedEntity.getProperty(referenceKey));
  				}
  			}
  
  			return properties;
  		}
  
  		return null;
  	}
docs: ----- interface PropertyGroup -----