code: private static void addRelationPropertyDefinitions(RelationshipTypeDefinitionImpl type) {
  
          type.addPropertyDefinition(
              createPropDef(
                  PropertyIds.SOURCE_ID,
                  "Source",
                  "Source",
                  PropertyType.ID,
                  Cardinality.SINGLE,
                  Updatability.ONCREATE,
                  false,
                  true));
  
          type.addPropertyDefinition(
              createPropDef(
                  PropertyIds.TARGET_ID,
                  "Target",
                  "Target",
                  PropertyType.ID,
                  Cardinality.SINGLE,
                  Updatability.ONCREATE,
                  false,
                  true));
  
      }
docs: Helper method to add the property definitions specific to relationship types.<p>
  
  @param type the type definition to which the property definitions should be added