schema.rb

ActiveRecord::Schema.define(:version => 20100824224246) do

  create_table "users", :force => true do |t|
    t.string   "name"
    t.text     "friend_list"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

end

seeds.rb

[
  {:name => 'Alice', :friend_list => '2,3' },
  {:name => 'Bob', :friend_list => '1' },
  {:name => 'Charlie', :friend_list => '2' }
].each do |u|
  User.create(u)
end

user.rb

class User < ActiveRecord::Base
  attr_protected :friend_list
  
  def following
    self.class.all(:conditions => "id in (#{friend_list})")
  end
  
  def follow(id)
    update_attribute(:friend_list, (friend_list.split(',').map(&:to_i) | [id.to_i]).join(','))
  end
  
  def unfollow(id)
    update_attribute(:friend_list, (friend_list.split(',').map(&:to_i) - [id.to_i]).join(','))
  end
  
  def followers
    self.class.all(:conditions => ['find_in_set(?, friend_list)', id])
  end
end