Rails自身のmany to many関係self has_many

# person people: id; name; whatever-you-need friendships: id; person_id; friend_id
class Person < ActiveRecord::Base
   has_many :friendships, :foreign_key => "person_id", 
      :class_name => "Friendship"

   has_many :friends, :through => :friendships
end

class Friendship < ActiveRecord::Base
   belongs_to :person, :foreign_key => "person_id", :class_name => "Person"
   belongs_to :friend, :foreign_key => "friend_id", :class_name => "Person"  
end

ActiveRecord::Schema.define(:version => 20100410040453) do

  create_table "people", :force => true do |t|
    t.string   "name"
    t.string   "sex"
    t.integer  "father_id"
    t.integer  "mother_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end

# source ， belongs_to 

class Person < ActiveRecord::Base
  self.inheritance_column = "sex"

  belongs_to :father, :class_name => "Person"
  belongs_to :mother, :class_name => "Person"
end

class Male < Person
  has_many :children, :class_name => "Person", :foreign_key => :father_id
  has_many :babymommas, :through => :children, :source => :mother, :uniq => true
end

class Female < Person
  has_many :children, :class_name => "Person", :foreign_key => :mother_id
  has_many :babydaddies, :through => :children, :source => :father, :uniq => true
end

create_table :users do |t|
  t.column :login, :string, :null => false
end
create_table :relationships, :id => false do |t|
  t.column "user_id",  :integer, :null => false
  t.column "buddy_id", :integer, :null => false
end

describe User do
  before :each do
    %w(u v w).each do |x|
      eval "@#{x} = User.new; @#{x}.login = '#{x}'; @#{x}.save"
    end
  end
  it "can be linked with another user" do
    @u.linked_to << @v
    @u.linked_to.should include(@v)
    u2 = User.find_by_id @u.id
    u2.linked_to.should include(@v)
  end
  it "can find users that linked to it" do
    @u.linked_to << @v
    @v.linked_from.should include(@u)
  end
  it "can see friends of its friends" do
    @u.linked_to << @v
    @v.linked_to << @w
    @u.linked_to[0].linked_to.should include(@w)
    @w.linked_from[0].linked_from.should include(@u)
  end
  it "has a list of all users linked to and from it" do
    @u.linked_to << @v
    @v.linked_to << @w
    @v.linked.should include(@u)
    @v.linked.should include(@w)
  end
end

class Relationship < ActiveRecord::Base
  belongs_to :user,
    :class_name => 'User', :foreign_key => 'user_id'
  belongs_to :buddy,
    :class_name => 'User', :foreign_key => 'buddy_id'
end
class User < ActiveRecord::Base
  has_many :relations_to,
    :foreign_key => 'user_id',  :class_name => 'Relationship'
  has_many :relations_from,
    :foreign_key => 'buddy_id', :class_name => 'Relationship'                             

  has_many :linked_to,
    :through => :relations_to,   :source => :buddy
  has_many :linked_from,
    :through => :relations_from, :source => :user
end

  select user_id, buddy_id
    from relationships
  union
  select buddy_id, user_id
     from relationships

def linked
  User.find_by_sql "
    select *
    from users
    where id in (
      select buddy_id
      from buddies
      where user_id = #{id})"
end