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

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.