Self Referential Associations with has_many :through

db/migrate/001_create_teletubbies.rb

class CreateTeletubbies < ActiveRecord::Migration
  def self.up
    create_table :teletubbies do |t|
      t.string :name
      t.timestamps
    end
    Teletubby.create(:name => 'Tinky Winky')
    Teletubby.create(:name => 'Dipsy')
    Teletubby.create(:name => 'Laa Laa')
    Teletubby.create(:name => 'Po')
  end

  def self.down
    drop_table :teletubbies
  end
end

db/migrate/002_create_friendships.rb

class CreateFriendships < ActiveRecord::Migration
  def self.up
    create_table :friendships do |t|
      t.integer :teletubby_id
      t.integer :friend_id
      t.timestamps
    end
    # Tinky Winky loves all the other Teletubbies very much
    Friendship.create(:teletubby_id => 1, :friend_id => 2)
    Friendship.create(:teletubby_id => 1, :friend_id => 3)
    Friendship.create(:teletubby_id => 1, :friend_id => 4)
    # Dipsy loves all the other Teletubbies very much
    Friendship.create(:teletubby_id => 2, :friend_id => 1)
    Friendship.create(:teletubby_id => 2, :friend_id => 3)
    Friendship.create(:teletubby_id => 2, :friend_id => 4)
    # Laa Laa loves Po very much
    Friendship.create(:teletubby_id => 3, :friend_id => 4)
    # Po loves Laa Laa very much
    Friendship.create(:teletubby_id => 4, :friend_id => 3)
    # Big Hug
  end

  def self.down
    drop_table :friendships
  end
end

app/models/teletubby.rb

class Teletubby < ActiveRecord::Base
  # a teletubby 'has_many' friendships (teletubbies love each other very much)
  has_many :friendships

  # a teletubby 'has_many' friends ':through' friendships relationship
  has_many :friends, :through => :friendships
end

app/models/friendship.rb

class Friendship < ActiveRecord::Base
  # ActiveRecord already knows what a teletubby is (reflection)
  belongs_to :teletubby

  # No actual friendship model, so add :class_name and :foreign_key
  belongs_to :friend, :class_name => "Teletubby", :foreign_key => "friend_id"
end

The database should look a little something like this:

mysql> select * from teletubbies;
+----+-------------+---------------------+---------------------+
| id | name        | created_at          | updated_at          |
+----+-------------+---------------------+---------------------+
|  1 | Tinky Winky | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  2 | Dipsy       | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  3 | Laa Laa     | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  4 | Po          | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
+----+-------------+---------------------+---------------------+
4 rows in set (0.00 sec)

mysql> select * from friendships;
+----+--------------+-----------+---------------------+---------------------+
| id | teletubby_id | friend_id | created_at          | updated_at          |
+----+--------------+-----------+---------------------+---------------------+
|  1 |            1 |         2 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  2 |            1 |         3 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  3 |            1 |         4 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  4 |            2 |         1 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  5 |            2 |         3 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  6 |            2 |         4 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  7 |            3 |         4 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
|  8 |            4 |         3 | 2008-07-21 15:41:42 | 2008-07-21 15:41:42 | 
+----+--------------+-----------+---------------------+---------------------+
8 rows in set (0.00 sec)

Use script/console to check the relationships:

dsparling-imbps-computer:~/Sites/teletubbies dsparlingimbp$ script/console 
Loading development environment (Rails 2.0.2)
>> t1 = Teletubby.find(1)
=> #<Teletubby id: 1, name: "Tinky Winky", created_at: "2008-07-21 15:41:42", updated_at: "2008-07-21 15:41:42">
>> t1.friends.each { |f| puts "#{t1.name} loves #{f.name} very much." }; false
Tinky Winky loves Dipsy very much.
Tinky Winky loves Laa Laa very much.
Tinky Winky loves Po very much.
=> false
>> t2 = Teletubby.find(2)
=> #<Teletubby id: 2, name: "Dipsy", created_at: "2008-07-21 15:41:42", updated_at: "2008-07-21 15:41:42">
>> t2.friends.each { |f| puts "#{t2.name} loves #{f.name} very much." }; false
Dipsy loves Tinky Winky very much.
Dipsy loves Laa Laa very much.
Dipsy loves Po very much.
=> false
>> t3 = Teletubby.find(3)
=> #<Teletubby id: 3, name: "Laa Laa", created_at: "2008-07-21 15:41:42", updated_at: "2008-07-21 15:41:42">
>> t3.friends.each { |f| puts "#{t3.name} loves #{f.name} very much." }; false
Laa Laa loves Po very much.
=> false
>> t4 = Teletubby.find(4)
=> #<Teletubby id: 4, name: "Po", created_at: "2008-07-21 15:41:42", updated_at: "2008-07-21 15:41:42">
>> t4.friends.each { |f| puts "#{t4.name} loves #{f.name} very much." }; false
Po loves Laa Laa very much.
=> false