Draft schema for Twitter type followers, following functionality:
```ruby
create_table "followers", :force => true do |t|
t.integer "leader_id"
t.integer "follower_id"
end
add_index "followers", ["leader_id", "follower_id"], :name => "index_followers_on_leader_id_and_follower_id", :unique => true
```
Just an idea, haven't tested this yet.
Update: railscasts.com has published an article on how to use self-referential associations for this purpose:
http://railscasts.com/episodes/163-self-referential-association