Ancestry allows the records of a ActiveRecord model to be organized in a tree structure, using the materialized path pattern. It exposes the standard relations (ancestors, parent, root, children, siblings, descendants) and allows them to be fetched in a single query. Additional features include named scopes, integrity checking, integrity restoration, arrangement of (sub)tree into hashes and different strategies for dealing with orphaned records.

An awesome nested set implementation for Active Record

Easily and efficiently make your ActiveRecord model support hierarchies

A gem that adds simple support for organizing ActiveRecord models into parent–children relationships.

NOTICE: official repository moved to https://github.com/bbommarito/acts_as_nested_set

better_nested_set Rails plugin (my fork from official SVN)

ActiveRecord extension for sorted adjacency lists support

Arboreal is yet another extension to ActiveRecord to support tree-shaped data structures. Internally, Arboreal maintains a computed "ancestry_string" column, which caches the path from the root of a tree to each node, allowing efficient retrieval of both ancestors and descendants. Arboreal surfaces relationships within the tree like "children", "ancestors", "descendants", and "siblings" as scopes, so that additional filtering/pagination can be performed.

Adds directed acyclic graph functionality to ActiveRecord.