self referential m:m relationships

gistfile1.rb

class Category
    include DataMapper::Resource

    property :id, Serial
    # some other properties here

    has n, :user_children, 'Categoryship', child_key: [ :source_id ]
    has n, :user_parents,  'Categoryship', child_key: [ :target_id ]

    has n, :children, self, through: :user_children, via: :target
    has n, :parent, self, through: :user_parents, via: :source
  end

  class Categoryship
    include DataMapper::Resource

    belongs_to :source, 'Category', key: true
    belongs_to :target, 'Category', key: true
  end

gistfile2.sh

[4] pry(main)> Model::Category.first(main: true).children
=> []
[5] pry(main)> Model::Category.first(main: true).parent
=> [#<Model::Category @id=3 @name=<not loaded> @slug=<not loaded> @main=false @created_at=#<DateTime: 2013-02-23T11:32:40+00:00 ((2456347j,41560s,0n),+0s,2299161j)> @created_on=#<Date: 2013-02-23 ((2456347j,0s,0n),+0s,2299161j)> @updated_at=#<DateTime: 2013-02-23T11:35:36+02:00 ((2456347j,34536s,0n),+7200s,2299161j)> @updated_on=#<Date: 2013-02-23 ((2456347j,0s,0n),+0s,2299161j)>>]