code: public void updateRelationshipIndex(EntityMetadata entityMetadata, GraphDatabaseService graphDb,
              Relationship relationship, MetamodelImpl metaModel)
      {
          if (!isRelationshipAutoIndexingEnabled(graphDb) && entityMetadata.isIndexable())
          {
              Index<Relationship> relationshipIndex = graphDb.index().forRelationships(entityMetadata.getIndexName());
  
              // Remove all existing relationship entries from Index
              relationshipIndex.remove(relationship);
  
              // Recreate fresh index on this relationship
              addRelationshipIndex(entityMetadata, relationship, relationshipIndex, metaModel);
          }
      }
docs: If relationship auto-indexing is disabled, Update index for this
  relationship manually
  
  @param entityMetadata
  @param graphDb
  @param autoIndexing
  @param node