ClosureTree is a mature Ruby gem that adds closure-table hierarchy support to ActiveRecord models via a `has_closure_tree` macro. It maintains a separate `_hierarchies` table storing ancestor/descendant pairs with generation depth, enabling O(1) SELECTs for ancestors, descendants, siblings, leaves, and roots. The gem handles the full lifecycle: node creation inserts a self-referencing row plus copies of the parent's ancestor rows (2 INSERTs), reparenting rebuilds the affected hierarchy, and destruction cleans up references with configurable dependent behavior (nullify, adopt, destroy, delete_all). It supports STI, UUID primary keys, deterministic numeric ordering with preorder traversal, scoped multi-tenant trees, PostgreSQL schema-qualified tables, and multi-database setups (PostgreSQL/MySQL/SQLite simultaneously). Concurrency safety is provided through the with_advisory_lock gem with customizable lock names and per-instance lock granularity for multi-tenant use cases. The architecture is notably clean: a `ClosureTree::Support` instance (`_ct`) encapsulates all implementation details to avoid polluting the host model's namespace, and focused mixins (Finders, HashTree, Digraphs, DeterministicOrdering, HierarchyMaintenance) are composed by the `has_closure_tree` entry point. The code demonstrates sophisticated cross-database SQL engineering — MySQL uses user variables for gap-free sibling reordering, PostgreSQL uses window functions, and Arel query builders handle adapter-specific quoting. The test suite is exceptional: it runs against Ruby 3.4/4.0, Rails 7.2 through edge, and all three database engines, with concurrency tests, query-count assertions, and shared example suites for both integer and UUID primary keys. The gem is actively maintained with release-please automation, semantic versioning, and a detailed changelog spanning over a decade of development.