Arboreal is yet another extension to ActiveRecord to support tree-shaped data structures.
Arboreal surfaces relationships within the tree like children, ancestors, descendants, and siblings as scopes, so that additional filtering/pagination can be performed.
It delegates as much work as possible to the underlying DBMS, making it efficient to:
fetch all ancestors, descendants or siblings of a node
move nodes (or subtrees) around
rebuild the hierarchy
First, install the “arboreal” gem, and add it to your Rails project's config/environment.rb.
Next, you'll need a migration to add parent_id and ancestry_string columns, and indices:
class MakeThingsArboreal < ActiveRecord::Migration def self.up add_column "things", "parent_id", :integer add_index "things", ["parent_id"] add_column "things", "ancestry_string", :string add_index "things", ["ancestry_string"] end def self.down remove_index "things", ["ancestry_string"] remove_column "things", "ancestry_string" remove_index "things", ["parent_id"] remove_column "things", "parent_id" end end
Finally, you can declare your model arboreal:
class Thing < ActiveRecord::Base # .. etc etc ... end
Navigating the tree
Arboreal adds the basic relationships you'd expect:
In addition, it provides the following handy methods on each tree-node:
subtree (the node itself, plus descendants)
root (the topmost ancestor)
The first four return scopes, to which additional filtering, ordering or limits may be applied.
At the class-level:
roots is a named-scope returning all the nodes without parents
rebuild_ancestry rebuilds the ancestry cache, as described below
Rebuilding the ancestry cache
Internally, Arboreal uses the ancestry_string column to cache the path down the tree to each node (or more correctly, it's parent. This technique - a variant of “path enumeration” or “materialized paths” - allows efficient retrieval of both ancestors and descendants.
It's conceivable that the computed ancestry-string values may get out of whack, particularly if changes are made directly to the database. If you suspect corruption, you can restore sanity using rebuild_ancestry, e.g
The ancestry rebuild is implemented in SQL to leverage the underlying DBMS, and so is pretty efficient, even on large trees.