Example of friendships and retriving all records.

friendship_example.rb

#!/usr/bin/env ruby
#
# Example of friendships and retriving all records.
require 'rubygems'
require 'dm-core'
require 'dm-migrations'


# setup the logger
DataMapper::Logger.new($stdout, :debug)

# connect to the DB
DataMapper.setup(:default, 'sqlite3::memory:')

class Person
  include DataMapper::Resource

  property :id,    Serial
  property :name , String, :required => true

  def all_friends
    self.class.all('friendships.source_id' => self.id) | self.class.all('friendships.target_id' => self.id)
  end



  has n, :friendships, :child_key => [ :source_id ]
  has n, :friends, self, :through => :friendships, :via => :target
end

class Friendship
  include DataMapper::Resource

  belongs_to :source, 'Person', :key => true
  belongs_to :target, 'Person', :key => true
end


DataMapper.finalize.auto_migrate!


@alice     = Person.create(:name => 'Alice')
@bob       = Person.create(:name => 'Bob')
@charlotte = Person.create(:name => 'Charlotte')
@david     = Person.create(:name => 'David')
@eve       = Person.create(:name => 'Eve')

@alice.friends << @bob
@alice.save

p @alice.friends
p @bob.friends
p @bob.all_friends

@charlotte.friends << @david
@charlotte.save
@eve.friends << @david
@eve.save
@david.friends << @bob
@david.save

p @david.all_friends