>>380
こんなのパターンかい?

A: customer(id, name)
B: subscription(id, customer_id, magazine_id, paid, expire_at)
C: magazine(id, title)

class Customer < AR::Base
has_many :subscriptions
end

class Magazine < AR::Base
has_many :subscriptions
end

class Subscription < AR::Base
belongs_to :customer
belongs_to :magazine
end