How do I calculate the most popular combination of a order lines? (or any similar order/order lines db arrangement) - ruby-on-rails

Question

I'm using Ruby on Rails. I have a couple of models which fit the normal order/order lines arrangement, i.e.
class Order
has_many :order_lines
end
class OrderLines
belongs_to :order
belongs_to :product
end
class Product
has_many :order_lines
end
(greatly simplified from my real model!)
It's fairly straightforward to work out the most popular individual products via order line, but what magical ruby-fu could I use to calculate the most popular combination(s) of products ordered.
Cheers,
Graeme

Answer 1

My suggestion is to create an array a of Product.id numbers for each order and then do the equivalent of
h = Hash.new(0)
# for each a
h[a.sort.hash] += 1
You will naturally need to consider the scale of your operation and how much you are willing to approximate the results.
External Solution
Create a "Combination" model and index the table by the hash, then each order could increment a counter field. Another field would record exactly which combination that hash value referred to.
In-memory Solution
Look at the last 100 orders and recompute the order popularity in memory when you need it. Hash#sort will give you a sorted list of popularity hashes. You could either make a composite object that remembered what order combination was being counted, or just scan the original data looking for the hash value.

Answer 2

Thanks for the tip digitalross. I followed the external solution idea and did the following. It varies slightly from the suggestion as it keeps a record of individual order_combos, rather than storing a counter so it's possible to query by date as well e.g. most popular top 10 orders in the last week.
I created a method in my order which converts the list of order items to a comma separated string.
def to_s
order_lines.sort.map { |ol| ol.id }.join(",")
end
I then added a filter so the combo is created every time an order is placed.
after_save :create_order_combo
def create_order_combo
oc = OrderCombo.create(:user => user, :combo => self.to_s)
end
And finally my OrderCombo class looks something like below. I've also included a cached version of the method.
class OrderCombo
belongs_to :user
scope :by_user, lambda{ |user| where(:user_id => user.id) }
def self.top_n_orders_by_user(user,count=10)
OrderCombo.by_user(user).count(:group => :combo).sort { |a,b| a[1] <=> b[1] }.reverse[0..count-1]
end
def self.cached_top_orders_by_user(user,count=10)
Rails.cache.fetch("order_combo_#{user.id.to_s}_#{count.to_s}", :expiry => 10.minutes) { OrderCombo.top_n_orders_by_user(user, count) }
end
end
It's not perfect as it doesn't take into account increased popularity when someone orders more of one item in an order.