Arboreal

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

• prevent loops

• rebuild the hierarchy

Getting started

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

  acts_arboreal

  # .. etc etc ...

end

Navigating the tree

Arboreal adds the basic relationships you’d expect:

• parent

• children

In addition, it provides the following handy methods on each tree-node:

• ancestors

• descendants

• subtree (the node itself, plus descendants)

• siblings

• 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

Thing.rebuild_ancestry

The ancestry rebuild is implemented in SQL to leverage the underlying DBMS, and so is pretty efficient, even on large trees.