Self Referential Model

entity.rb

class Entity
  include MongoMapper::Document
  safe

  before_destroy :remove_relations

  key :relation_ids, Array
  key :name, String
  many :relations, :in => :relation_ids, :class => Entity

  def related
    relations + Entity.where(:relation_ids => _id).all
  end

  def self.create_indexes
    ensure_index :relation_ids
  end

  private
    def remove_relations
      Entity.pull({},:relation_ids => id)
    end
end