What are the associations between the models in a subscription/recurring billing Rails app?

This is how the schema for the four models look: http://pastie.org/1576759

The plan table stores all the data on the actual plans. The subscription stores each month that a user 're-s开发者_如何学Cubscribes' to the service. The transaction stores the payment related info.

How would the associations work between models ?

E.g. a user :belongs_to plan, :through => :subscription ?

A subscription "has_many" :plans ?

I am a bit fuzzy on how this all ties together with respect to Rails and associations.

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

class User < ActiveRecord::Base
  belongs_to :plan
  has_many :subscriptions (or has_one, if a user only has 1 subscription at a time)
  has_many :transactions
end

class Transaction < ActiveRecord::Base
  belongs_to :user
  belongs_to :plan
end

class Plan < ActiveRecord::Base
  has_many :subscriptions
  has_many :users
end