Acts As Ordered Tree

class CreateCategories < ActiveRecord::Migration
  def self.up
    create_table :categories do |t|
      t.integer :company_id
      t.string  :name
      t.integer :parent_id # this is mandatory
      t.integer :position # this is mandatory
      t.integer :depth # this is optional
      t.integer :children_count # this is optional
    end
  end

  def self.down
    drop_table :categories
  end
end

class Category < ActiveRecord::Base
  acts_as_ordered_tree

  # gem introduces new ActiveRecord callbacks:
  # *_reorder - fires when position (but not parent node) is changed
  # *_move - fires when parent node is changed
  before_reorder :do_smth
  before_move :do_smth_else
end

# root
#  \_ child1
#       \_ subchild1
#       \_ subchild2


root = Category.create(:name => "root")
child1 = root.children.create(:name => "child1")
subchild1 = child1.children.create("name" => "subchild1")
subchild2 = child1.children.create("name" => "subchild2")

Category.roots # => [root]

root.root? # => true
root.parent # => nil
root.ancestors # => []
root.descendants # => [child1, subchild1, subchild2]
root.descendants.arrange # => {child1 => {subchild1 => {}, subchild2 => {}}}
# you may pass an option to discard possible orphans from selection
root.descendants.arrange(:orphans => :discard)

child1.parent # => root
child1.ancestors # => [root]
child1.children # => [subchild1, subchild2]
child1.descendants # => [subchild1, subchild2]
child1.root? # => false
child1.leaf? # => false

subchild1.ancestors # => [child1, root]
subchild1.root # => [root]
subchild1.leaf? # => true
subchild1.first? # => true
subchild1.last? # => false
subchild2.last? # => true

subchild1.move_to_above_of(child1)
subchild1.move_to_bottom_of(child1)
subchild1.move_to_child_of(root)
subchild1.move_lower
subchild1.move_higher