ClosureTree/closure_tree: Easily and efficiently make your ActiveRecord models support hierarchies

Dramatically more performant than ancestry and acts_as_tree, and even more awesome than awesome_nested_set, closure_tree has some great features: Best-in-class select performance: Fetch your whole ancestor lineage in 1 SELECT. Grab all your descendants in 1 SELECT. Get all your siblings in 1 SELECT. Fetch all descendants as a nested hash in 1 SELECT. Find a node by ancestry path in 1 SELECT. Best-in-class mutation performance: 2 SQL INSERTs on node creation 3 SQL INSERT/UPDATEs on node reparenting.