has_many and has_many :through?

gistfile1.rb

class User < ActiveRecord::Base
  has_many :subscriptions
  has_many :threads, :through => :subscriptions
  has_many :threads
end

class Thread < ActiveRecord::Base
  has_many :subscriptions
  has_many :users, :through => :subscriptions
  belongs_to :user
end

class Subscription < ActiveRecord::Base
  belongs_to :thread
  belongs_to :user
end