Specify this act if you want to model a tree structure by providing a parent association and a children association. This act requires that you have a foreign key column, which by default is called parent_id.

  class Category < ActiveRecord::Base
    acts_as_tree :order => "name"
  end

  Example :
  root
   \_ child1
        \_ subchild1
        \_ subchild2

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

  root.parent   # => nil
  child1.parent # => root
  root.children # => [child1]
  root.children.first.children.first # => subchild1

In addition to the parent and children associations, the following instance methods are added to the class after specifying the act:

  • siblings : Returns all the children of the parent, excluding the current node ([ subchild2 ] when called from subchild1)
  • self_and_siblings : Returns all the children of the parent, including the current node ([ subchild1, subchild2 ] when called from subchild1)
  • ancestors : Returns all the ancestors of the current node ([child1, root] when called from subchild2)
  • root : Returns the root of the current node (root when called from subchild2)
Methods
Public Instance methods
acts_as_tree(options = {})

Configuration options are:

  • foreign_key - specifies the column name to use for tracking of the tree (default: parent_id)
  • order - makes it possible to sort the children according to this SQL snippet.
  • counter_cache - keeps a count in a children_count column if set to true (default: false).
    # File vendor/rails/activerecord/lib/active_record/acts/tree.rb, line 43
43:         def acts_as_tree(options = {})
44:           configuration = { :foreign_key => "parent_id", :order => nil, :counter_cache => nil }
45:           configuration.update(options) if options.is_a?(Hash)
46: 
47:           belongs_to :parent, :class_name => name, :foreign_key => configuration[:foreign_key], :counter_cache => configuration[:counter_cache]
48:           has_many :children, :class_name => name, :foreign_key => configuration[:foreign_key], :order => configuration[:order], :dependent => :destroy
49: 
50:           class_eval "include ActiveRecord::Acts::Tree::InstanceMethods\n\ndef self.roots\nfind(:all, :conditions => \"\#{configuration[:foreign_key]} IS NULL\", :order => \#{configuration[:order].nil? ? \"nil\" : %Q{\"\#{configuration[:order]}\"}})\nend\n\ndef self.root\nfind(:first, :conditions => \"\#{configuration[:foreign_key]} IS NULL\", :order => \#{configuration[:order].nil? ? \"nil\" : %Q{\"\#{configuration[:order]}\"}})\nend\n"
51:         end