Ltree Hierarchy

A simplistic gem that allows ActiveRecord models to be organized in a tree or hierarchy. It uses a materialized path implementation based around PostgreSQL's ltree data type, associated functions and operators.

Why might you want to use it?

• You want to be able to construct optimized hierarchical queries with ease, both from Ruby AND raw SQL.
• You want to be able to compose complex arel expressions from pre-defined building blocks.
• You prefer PostgreSQL over other relational DBs.

Installation

Add this line to your application's Gemfile:

gem 'ltree_hierarchy'

And then execute:

$ bundle

Add ltree extension to PostgreSQL:

$ psql -U postgres -d my_database
-> CREATE EXTENSION IF NOT EXISTS ltree;

Update your table:

class AddLtreeToLocations < ActiveRecord::Migration
  def self.up
    add_column :locations, :parent_id, :integer
    add_column :locations, :path, :ltree

    add_index :locations, :parent_id
  end

  def self.down
    remove_index :locations, :parent_id
    remove_column :locations, :parent_id
    remove_column :locations, :path
  end
end

Run migrations:

$ bundle exec rake db:migrate

Usage

  class Location < ActiveRecord::Base
    has_ltree_hierarchy
  end

  root     = Location.create!(name: 'UK')
  child    = Location.create!(name: 'London', parent: root)
  subchild = Location.create!(name: 'Hackney', parent: child)

  root.parent   # => nil
  child.parent # => root
  root.children # => [child]
  root.children.first.children.first # => subchild
  subchild.root # => root

• .roots
• .leaves
• .at_depth(n)
• .lowest_common_ancestors(scope)
• #parent
• #ancestors
• #self_and_ancestors
• #siblings
• #self_and_siblings
• #children
• #self_and_children
• #descendants
• #self_and_descendants
• #leaves

TODO

• Better error message for circular references.
• Don't neglect i18n.