class Magazine < ActiveRecord::Base
  has_many :subscriptions
end

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

class User < ActiveRecord::Base
  has_many :subscriptions
  has_one :magazine, :through => : subscriptions, :conditions => ['subscriptions.active = ?', true]
end

@ryan.subscriptions #=> []
@ryan.magazine = Magazine.create(:name => 'Hustler')
@ryan.subscriptions #=> [<Subscription magazine_id: 1, user_id: 1 ...>]