Question

In my Rails (3.2) app, an Order has many LineItems. A LineItem has many LineItemPayments. A LineItemPayment has one Payment. (LineItems can potentially be payed for multiple times (subscriptions), which is why I have the join table there.)

I need to be able to query for order information from a payment record. I can get an array of orders via relationships, but I know they will always be the same order. Is there a way in Rails to set up the association to reflect this? If not, would it better to set up a method for retrieving the array and then picking the order out of that, or rather just storing the order_id with the payment and set up a direct relationship that sidesteps all this?

Answer 1

You’ll need to work with the orders collection and narrow it down accordingly per your own logic. Although you certainly ‘can’ add the order_id to the payment directly, that will denormalize your data (as a cache) which is only recommended when you start hitting performance bottlenecks in your queries – otherwise it’s asking for trouble in the area of data integrity:

class Payment < ActiveRecord::Base
  has_many :line_item_payments
  has_many :line_items, :through => :line_item_payments
  has_many :orders, :through => :line_items

  # use this to get the order quickly
  def order
    orders.first
  end

  # use this to narrow the scope on the query interface for additional modifications
  def single_order
    orders.limit(1)
  end
end

class LineItemPayment < ActiveRecord::Base
  belongs_to :line_item
  belongs_to :payment
end

class LineItem < ActiveRecord::Base
  belongs_to :order
  has_many :line_item_payments
end